I am assuming you are connecting to an external anchor controller. Ethernet over IP is only supported on controllers running the same versions of code. Make sure you WLAN controllers and anchor controllers have the same version of code on them. This should resolve the problem.
after your post I checked the mobility group configuration on all four controllers, and indeed, there was a inconsitency in the configuration. The WCS couldn't see the mismatch, because the settings between the WCS and the controllers were the same. As I looked on the controllers web pages i saw, that different mobility group memebers with various mobility group names were configured on every single controller, maybe caused by applying a new controller template, where i changed the mobility group name.
I started from scratch configuring the mobility groups and the syslog error messages were stopping.
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
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