Why do our RFID cards keep going back to Invalid status?

We operate a charging network consisting of one P30 x-Series (master) and five P30 c-Series (slave) wallboxes. This has worked perfectly for a good year now. All participants have activated the charging sessions at the wallboxes assigned to them with their RFID card and there have been no problems. Until at some point the first card suddenly became invalid, then the second... and so on. The cards gradually went to INVALID status in the whitelist visible in the backend. We then tried to reactivate the cards via the whitelist, but this was only successful for a short time. One day later INVALID again. We then tried to reactivate all the cards on the master via a restart (service button). This worked - although we did not hear the ascending tone sequence described above, but only a simple acknowledgement tone - and in any case all the cards were displayed again in the whitelist (previous data, names, assigned wallboxes were deleted). We then used the whitelist to limit the assigned wallboxes again (not everyone should be able to charge everywhere) and now the first card is INVALID again. We have dated the validity of the changes to 2035 and/or 2050... but after one day they drop out for no apparent reason. Please help, because our charging network is not usable. Thank you!