We have an on-premises installation and we are having problems trying to manage IT Users. When we try to Edit or Create a new user, nothing happens after clicking "Save". No error messages, no validation errors, nothing.

I'm not sure if this is related, but if I go to the admin user's activity log, I can see this error message:

Failed on accessing potential blocked user with username 'test1' information from 'LifeTime' environment: System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> ssServiceCenter.ExcptGeneralError: LifeTime component not licensed.
   at ssServiceCenter.WebServices.PlatformServices_v8_0_0.User_GetLocks(String inWSUserName, String inWSPassword, String inWSUserExternalId, WORCCS_LoginAttemptRecord[]& outWSLocks, Boolean& outWSUnknownUser, Int32& outWSErrorCodeId, String& outWSErrorMessage)
   --- End of inner exception stack trace ---

It might also be helpful to mention that we have recently upgraded from P9 to P10.

Has anyone ever experienced this?

I don't, but this seems the right time to contact OutSystems Support.