today we had a lot of lag the whole day. Atm it got almost unplayable for me (block/chest/chat lag 30-60sec)
All people online noticed the lag so i guess it's not on my end. But we don't know it's cause.

Generally speaking i guess the same rules apply to FTB like tekkit? As in turn off all machines when you log out and use overflow prevention for buildcraft pipes?
Are there specifics numbers how much lag mythcraft ages cause? Or a limit per player how many to create?

The lag was due to a FTB bug in XyCraft which eventually led to a crash. Developer error.

The server has been restarted.

The sever was also due to restart via the server script, however was aborted due to the non-responsive FTB server. Xycraft was trying to create entities in non-existent chunks via a loop, the vanilla 'minecraft' code didn't like it and more or less freaked out with console spam, having to print a full stacktrace of errors (walls of text) to a console takes alot more resources in comparison to not displaying anything, thus caused the lag.

Turns out we might have a corrupted map too.

Is this what is stopping me from logging in at all?

Vanilla/Tekkit should be back up now. Working on FTB now.

FTB is now back up too.