Resonance problems after upgrading with Beacon and EBB42

I recently upgraded my stock VC3.1 400 with a Beacon and a EBB42 toolboard. but after this I can't get back to decent IS graps anymore.

In the X-axis IS graph there's a huge peak at the end of the measurement that did not exist before.

In the Y-axis IS graph there an additional peak at ~67Hz with a wide tail at the higher frequencies.

In the belt-tension graph there's a double peak for the lower belt instead of a single one around 60Hz.

I'm using the accelerometer on the Beacon board.

I have disassembled and reassembled more or less the entire print head and when reassembling I also took the opportunity to add thread locker to all screws and re-tighten everything.

Before upgrading, I did experiment a bit with higher belt tension but I didn't go to crazy with that.
Would a broken idler or similar appear like this in the graphs?

Any other ideas on what could be wrong?