Hi DeadDuck,
Since Microsoft Employees are not allowed to download the log file from Community for analysis. I could share some troubleshooting steps for your reference.
1. Start Fiddler and start capturing when you attempt to sign-on to a CRM organization using a browser(Ensure the sign-on is successful)
2. Check your trace log's Response Headers and log details under '<tr class="details">' like 'website has been blocked by mistake'.
Sometimes customer's network may have been blocked certain sites from being accessible (firewall rules). You need to work with your IT department to unblock CRM web sites like XXXXXX.crm.dynamics.com in order to use this tool on that network if it is.


Otherwise pls raise ticket in Service request and ask Dev Team to help invesitgate your trace log details and PluginRegistration.exe debug log.
Hope above would help.
Regards
Johnny