I'm sorry to hear about the problems you've encountered with your Sitefinity project.
Can you please let us know if there have been any modifications to the project's security configuration or its web.config related to membership and role providers sections?
In general the received exception indicates that somewhere you have instantiated a UserManager with the
"MYDOMAIN" provider name, for example UserManager.GetManager("MYDOMAIN").
Please check for a code that would be executing on a global application level (e.g. Global.asax, Sitefinity module Initialize(), custom HttpModule, or handler), which might be causing this behavior.
Alternatively, if you have not implemented anything similar to the above, and you do not ahve ocurrences similar to the UserManager.GetManager("MYDOMAIN") example, then can you please elaborate if you have indeed registered a membership provider in your system named MYDOMAIN, and whether this is the default provider. If so, please verify you have registered the provider correclty, or alternatively comment it out, so you can login in to the site backend while we're discussing the possible problems with its registration here.
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