Same problem here in SolidCP 1.4.0

only the solution from Cristi works for me!

just give the identity user of the application pool of the enterprise server the right . make shoe it use active directory account

Same here / version 1.4.1.
Adding accounts works flawlessly. checked the identity. that's an AD account.
Only deleting gives the error.

Maybe it's good to double check your hosting plan settings: Are you using the "Deleted users" function? if so i would recommend to uncheck that.

I had the same exact issue.

Found out that the Authentication Mode in the Active Directory Settings in SolidCP need to be set to "Secured". After that restart the AppPool and it should work as it should.