RE: User Login issues Guid should contain 32 digits with 4 dashes CRM 2013 on prem
Hey Crumbs (as I hope you don't mind being called),
The more detailed error is present under the "ArgumentOutOfRange" block with the "Expected UniqueId..." line.
Are you able to copy that entire line and paste it in? I can see that the URL the CRM is trying to load has "main.aspx?etc=4201&id=... but can't read the rest. It seems to be trying to load in the appointment entity (etc 4201 = appointment) but don't know much more than that based on the screenshot.
Please send the remainder of the error and I'll hopefully be able to get a resolution for you!
Thanks!
Matt Bayes