Failure creating new user with "FBA User Management"


Creating a FBA User used to work just fine. Recently however, creating a user resulted in the creation of two users: one called "AspNetSqlMembershipProvider:<USERID>" and second one called ":<USERID>". This is somewhat annoying but at least I could still create users (I just delete the second one and continue working with the first one).
But since today, for a reason not known to me, creating a user only generates the faulty user ":<USERID>" an no longer the correct user "AspNetSqlMembershipProvider:<USERID>". So now I can no longer create new FBA users.
Many thanks to anyone willing to help me solve this problem!
I've attached a screenshot demonstrating the problem...

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