I noticed since updating from 144 to 145 whenever I saved my game and quit to main menu my world got corrupted.

I did not have that issue on 144 so it must be one of the recent changes.

Gives error about backup level.dat being used.

Forge Mod Loader detected that the backup level.dat is being used.

This may happen due to a bug or corruption, continuing can damage
your world beyond repair or lose data / progress.

It's recommended to create a world backup before continuing.

It's actually not in the log files but actual ingame Minecraft warning message when trying to load world.

If I do hit yes on it it will load and it will either instantly crash or id's turn to mess.

Have you updated any other mods?
Have you messed around with your world in MCEdit?
Have you had a crash before where it might not have been able to save the world properly?

could you provide a copy of your log with that error please?

Closing for now, feel free to provide more info and I'll reopen. Just to trim out how many issues there are.

Have you updated any other mods?
No, only Botania, I tested by removing any mod I had updated before Botania.

Have you messed around with your world in MCEdit?
Nope

Have you had a crash before where it might not have been able to save the world properly?
Nope, not with Botania, but other mod had similiar issue, it had something to do with modular systems mod, I had to remove it from my pack because it kept corrupting my worlds and I coudn't fix it.

Since first incident with world corruption bug I've used Aromabackup to keep regular backups so I can replicate it corrupting pretty reliably.

https://drive.google.com/file/d/0B14v7mG8vy0Nc213SUMzVGdJbTQ/view?usp=sharing

Entire log file is 12 megabytes so using google drive instead. It is not causing minecraft to crash just corrupting world on saving.

Line 94040: [22:15:47] [Server thread/INFO] [FML/]: The state engine was in incorrect state SERVER_STARTING and forced into state SERVER_STOPPED. Errors may have been discarded.

This seems weird. The 145 changelog has nothing that could cause such a crash, all it does is that could break anything is add a new block. Maybe you ran out of IDs?

Post the whole log on gist or pastebin please.

I ran a dump of item and block id's, neither of them is anywhere close to the limit yet.

Could you try loading the 144 backup again and see if it works? Then just update to 145 and see if it breaks again.

I did that when I uploaded the log file, it's my current single player world, I have aromabackup making backups of it every 30 minutes or so. And yes, using 144 works.

See if it breaks if you try to update to 145 but not change any other mods.

It does.

Can you get a log from the time it gets the corruption message? Also what happens if you try to enter the world.

Log i linked earlier is when i try to load the corrupted world after saving it.

Hm, that's odd, I can't find any references to level.dat in the log. I'll post it around to see if someone can spot the issue better than me.

I can make version where I force it to load after warning message if that's any use.

Also to complete @collin1971 response, It seem that FML have decrement all block id on your map... That can be caused by Botania or something others...

I did some more testing with removing Underground biomes and installing Botania 146.

It's definitely issue with underground biomes as it works normally without it.