Hi all,
Thanks for thinking with me on this case.
We have solved the problem by deactivating & activating the mBGP sessions between the devices. After the sessions reestablished the EVPN functionality was working again.
We did not find anything during diagnosing. Resetting the BGP session was just guessing and trying everything.
We think we hit a bug during the updating and switching the active/inactive RE.
We are using 14.2R7.5 on MX960 with 32x10G and 2x100/8x10G linecards.
Allthough we have not tried to reproduce the issue (yet), i reccomend everything running EVPN features to upgrades thier devices by fully rebooting both RE's and not use features like Routing Engine Redundancy, Gracefull Routing Engine failover or similar functions. Better to go through the dark for a few minutes.