Technical info:
OS: Linux-Ubuntu-16.10
Java Version: openjdk-8 / openjdk-8-sdk
Forge Version: forge-1.11.2-13.20.0.2280
Optifine Version: 1.11.2_HD_U_B7

Observed that since you updated to your 3.0.0 version of SimplyConveors, it does not play nice with my heavily modded Minecraft. When loading an existing world (which was generated with the 2.1.2 version of your mod existent) you will be greeted with first a warning of Forge that youre loading a world on its backup data file.

Needless to say im generating this with having beforehand made a backup of my world. When you then continued to load the world, it was painfully low on FPS and in the last version it even crashes straight away. Comparison between having your mod 2.1.2 Version (25-30 FPS) and with >= 3.0.0 (0-2 FPS or crash).

I wasn't able to load into version 3.0.2 at all, so I decided to write you this and collect all log data I could get as clean I could get it (removed all logs before start so i only got the logs of this particular run which I will attach following this text.

Crash Report: crash-2017-04-10_12.41.41-server.txt (was not generating a client side at all)
Better Foliage Log: betterfoliage.txt
Custom NPCs Log: CustomNPCs-latest.txt
FML Client latest Log: fml-client-latest.txt
FML Junk earlystartup Log: fml-junk-earlystartup.txt
Latest Log: latest.txt
Ruins Log Files for Dimensions:
ruins_log_dim_0.txt
ruins_log_dim_1.txt
ruins_log_dim_-1.txt
ruins_log_dim_-2.txt
ruins_log_dim_-3.txt
ruins_log_dim_-9999.txt
ruins_log_dim_-11325.txt

Ah don't worry bout time, I extra converted to 3.0.02 just to report this, will stick with 2.1.2 for now, but unless you know you can't do anything about this issue. And i love Minecraft with mods :)

I just hope you got enough information out of this, if you need testing just let me know.

Your 2.1.2 version does work with all mods listed in the crash report as far I was able to test it yet. here the log file of a complete sane boot up.

betterfoliage.txt
CustomNPCs-latest.txt
fml-client-latest.txt
fml-junk-earlystartup.txt
latest.txt
ruins_log_dim_0.txt
ruins_log_dim_1.txt
ruins_log_dim_-1.txt
ruins_log_dim_-2.txt
ruins_log_dim_-3.txt
ruins_log_dim_-9999.txt
ruins_log_dim_-11325.txt

I'll do some testing when I get home later, but at any rate it'll be at least 8 hours :/. Sorry about that.

All I can say is just stick with 2.1.2 for a bit longer until I can resolve the error.

I'm back home now. I think I know what the issue is, just have to test my theory.

I'm afraid I have legitimately no clue what is going on. I'll look into it some more over the next few days.

The good news is making new worlds should work totally fine once it's time for you to make a new one :P.

O.k. this works without crash and FPS slowdown. Beside from the message that some items are missing nothing bad happened on world load.

Logs:
betterfoliage.txt
CustomNPCs-latest.txt
fml-client-latest.txt
fml-junk-earlystartup.txt
latest.txt
ruins_log_dim_0.txt
ruins_log_dim_1.txt
ruins_log_dim_-1.txt
ruins_log_dim_-2.txt
ruins_log_dim_-3.txt
ruins_log_dim_-9999.txt
ruins_log_dim_-11325.txt

Try this build of the Mod and let me know if it still occurs.

Well this is somewhat bad news. The good news is it no longer happens, but the bad news is it was happening because I was trying to replace old items with the new items.

This is good enough though. I'll just stick a disclaimer saying people should be aware that they /will/ lose all their conveyors.

Sad to hear it is causing so much trouble. On the other side not everyone runs his game this heavily modded as I do, guess I just have to much time on my hands. Maybe its just an other mod interfering, but I would have no clue how to pinpoint that without doing a whole day long nothing else as starting Minecraft and switching through mods.

Thanks, reverting back to 2.1.2 fixes the issue. I guess ATM2 will stay on that version for the foreseeable future. :)

@whatthedrunk Yeah, sorry about that. The best fix I've found so far is just to remove the code which replaces old items with newer versions, but that's a bad hack at best. I'm trying to find ways to get around this and I have a few I plan to try.

My current idea is just to have older blocks convert into 'Upgrade Crates' which replace the blocks with crates which upon being right-clicked will convert to the new version of the block.

This update caused some scary moments, as it somehow cascaded into some other mods acting up and the world appearing acting corrupted. After seeing SC appear in the missing ID error list, downgrading it fixed it.

I'll try your build you linked later, what is the behavior compared to 3.0.2?

@Dykam make sue you backup before trying his test build he posted here. Because it acts as if you removed Simply conveyors, and installed Simply conveyors the same time, all blocks he has changed will vanish and you get no world corruption, but a list of missing blocks which may result in vanishing blocks from your world, you would have to replace/rebuild them with the new blocks introduced.

My advice test if you must, but backup. If not stick with 2.1.2 for now until he got a moment to fix this issue properly.

@Dykam I'm very sorry to hear that. I'm working hard at a fix but literally nothing wants to cooperate for some reason. Like Videonauth said you will lose all existing conveyors with the test version I have linked.

Well the good news is I have it replacing the item forms of the blocks, but not the blocks themselves now. I'll fix the blocks themselves tomorrow (hopefully) and finally release 3.0.3.

Well I have great news. As stated in #28, as of 3.0.3 all conveyors excluding the following: Grabber Conveyors, Detector Conveyors, Trap Door Conveyors, Mob Conveyors (their functionality is in all conveyors now), and Downwards Conveyor Ramps will stay in the world in both block and item form.

However, the conveyors will be in the form of Upgrade Crates. These crates can either be broken or right-clicked to drop their contents. In item form these crates can be right-clicked to open individually, or shift right-clicked to open all of the stack at once. They contain the updated forms of the older blocks.

This is due to the fact that since special conveyors have to be converted to modular conveyors (which use modules), I have to give both the conveyor itself and the module. Other side effects from the update from 1.4.1 to 3.0.3 include: Stair Conveyors rotating to the left, Vertical Conveyors turning themselves around, and Fast Conveyors (middle-tier) from previous versions will turn into the Fast Conveyor (top-tier) from new versions.

The update will be out in a couple of hours, maybe less, but I do have to backport to 1.10.2 quickly.

Here's an example of what the crates look like: http://puu.sh/voHim/36e8f31971.jpg

Also note, block forms of the crate will directly put the conveyors themselves onto the ground, but it will drop the modules as items. Some conveyors won't have upgrade crates because they stayed the same unlocalized name across versions.