Hendrik
There are a couple of known issues floating around that could manifest as this. What AOS version are you on and what topology (lms+bkup lms? rap? cap? bridge? tunnel?) do you have.
If it happens again, collect the following to help narrow it down faster
show ap remote debug mgmt-frames ap-name <>
show ap tech-support ap-name <>
logging level debugging user-debug <mac of affected client>
show auth-tracebuf
tar logs techsupport
regards
-jeff