Suddenly I keep getting this crash:

http://hastebin.com/cotefanuzo.pl

This must be because some internal Forge library isn't loaded for some reason.
Did you recently update Forge? Maybe something got corrupted, in which case you should try re-installing Forge.

I did actually. Now that you mention it. From 1387 to 1388, which completely removed the edited .jar file after installing forge prior 1387. As in there's no .jar anymore.

Guess it's back to 1387 again.

There were apparently some recent changes in FML (MinecraftForge/FML@d14d1a8) that could cause this issue. I'll leave this issue open for now, hopefully the FML change will be reverted or fixed soon.

It works reverting back to 1387. Cheers.

The breaking change has been reverted in FML (MinecraftForge/FML@7c10b93), so one of the next Forge releases should fix this issue.