Recently, I removed Open Blocks from my modpack. After that I wasn't unable anymore to load my world. The crash files are server files but I joined a single player world. Seems that removing OB corrupting the level.dat file though.

Before deleting Myscraft's stuff from save. See also: XCompWiz/Mystcraft-Issues#335
crash-2019-01-23_17.25.12-server.txt

After deleting Mystcraft's stuff.
crash-2019-01-23_19.32.56-server.txt

After adding back OB no crash appears.

Note that I tested different forge versions: 2795, 2796, 2808.

Does not seem to be related directly to OpenBlocks, seems like something is messing with Mystcraft. We are not doing anything with dimensions or do any hacks on persistent data.

I would suspect JustEnoughIDs or similar, they usually take mod removal poorly.

Do you know a way to solve this?

At this point? Probably no way to solve it. It's just a risk of using ID extenders.

Is OB adding Villagers?

Yes

Then it is mistake by GravestoneMod.
I found this in the log. Removing GravestoneMod solves the crash. The mod is creating corpse for each villager professions. Missing profession causes the null.

https://pastebin.com/qUyabbVT

That makes sense, thanks for the update.