Issue with jetbrains.exodus.bindings.ComparableSet while creating new user

I am an administrator of MyNameIstanceInCloud @ YouTrack and If I try to create a new user I am getting an error
'java.lang.String cannot be cast to jetbrains.exodus.bindings.ComparableSet'

This error started to appear today (29-Aug-2017) and nobody cannot even register to our YouTrack "incloud" instance. (even if till couple of days ago was possible).

I am not capable to understand what could be the reason since I did not (and my team as well) change anything in the settings of YouTrack. (NOTE: Nobody of our team can do this action to add a new user).

Thanks in advance for your help.

Regards.