Hi,
We can not diagnose the issue without going through the some outputs. if possible share the output of
1. show ap log system
2. show ap log security
3. show ap log user.
If you want to downgrade the IAP, take GUI access of the Master and follow these steps,
1. Navigate to Maintenance>Firmware. The Firmware window is displayed.
2. Under Manual section, Select the Image file option. This method is only available for single-class IAPs.
Note : you should select proper image file based on the class of your IAP.
Please feel free for any further help on this.