There was a bug in Sitefinity login widget for an endless loop when maximum number of concurrent users was reached (this is fixed in our latest release).
In case you upgrade your project, please follow our upgrade documentation
and If the problem persists, please open a support ticket to further investigate this issue.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items