Associated clients (C5) become 'deaf'

Since a couple of weeks (after last upgrade) we found that several times (random) several C5 client units from A5’s are associated (often after a power cut in the grid) but are unable to communicate with the network or A5.
In the client list they only show their mac addres but not name info etc. Clients report ‘they have no internet’.
Only way to solve is to reboot the A5 (which then takes some 10 minutes to have all clients back associated) which interrupts traffic for all associate clients.

This happens now on an almost daily base. We are in contact with the helpdesk and they even have access to one of the A5’s but after some weeks not a lot of progress is made…

I was wondering if there where more people with the same issue.