Hi Joe,
From the screenshot, I noticed the error is related to contacting the Server.API endpoint.
I had a similar issue a while ago while upgrading from 10 to 11. In my case I wasn't able to login in Service Center after clicking 'Apply and Exit', but that was also caused by not being able to reach the Server.API endpoint.
You might want to try the solution mentioned in this post. The instruction came from OutSystems support and solved the issue for me.
In any case, it is always a good idea to open a support case for these kind of issues as it will enable OutSystems to continue to improve the installation/upgrade process.
Regards,
Nordin