A customer of mine reported a problem editing Security Accounts and associating credentials with account, or account to roles.
I had a look at the customer SMP logs and found that it was full of SqlExceptions.
We had a remote session on this today and found that the problem comes from SQL right for the account, which are not sysadmin on the SQL Server and could not read data in sys.syslockinfo, and this caused any call to spResourceAssociationsLoadData to fail.
Granting read on the sys.syslockinfo resolves the issue.
With that said, good week-end Connect!