In the case that you can't via the browser, but have access via cli, you can issue the following from the command given by Anusha in the debug menu of the ZD:
rksap_cli -A "set http/https enable"
Note: -A is to issue the command to all AP's. If you want only specific AP's use the -a command:
rksap_cli -a aa:bb:cc:dd:ee:ff "set http/https enable"
Note: on certain firmware version you have to use single quote as opposed to double quotes when issuing the rksap_cli command form the ZD
I should also have mentioned that this issue management issues (both web and ssh) predominantly effected MESH AP's, and in my experiences the reboot solved the problem in 100% of my cases.