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Looks like there is a bug in the Active Directory provider. We can't find any distinguished names in your connection string, but we don't check for that, and that's why it crashes.
Could you try with a connection string in this format and see if it works:
We are using Active directory Role and Membership providers for one of our websites although when we specify the Active directory using the GC: moniker we get the following error message (using LDAP within the connectionString works fine) :
Parser Error Message: Index was out of range. Must be non-negative and
less than the size of the collection.
Parameter name: index
Is anyone able to assist with this ?