I has Altaro PSB running nicely on my old 2008 R2 box for a couple of days, but this morning I am greeted by this message on the console when I attempt to log in:

Your session has expired

 You have been Signed Out. Please Sign In again to continue using Altaro Physical Server Backup.

When I attempt to log in again, the green "Sign In" button is dark green/disabled.  (Here I have filled the form with anonymous inputs for security)

I have restarted the service, which allows me to attempt to log in again, only to get the same session expired message.  I performed a repair using the original installation package, with no change.  I can't reboot the server as it's a live production box.  Any ideas?

Sure, I can do that.  Meanwhile I just found the following on the log, looks like the service is not starting on boot and throws this error.  Maybe this is all we need to see?

Faulting application name: Altaro.PhysicalBackup.Service.exe, version: 1.1.0.6, time stamp: 0xf57a0bf7
Faulting module name: KERNELBASE.dll, version: 6.1.7601.24511, time stamp: 0x5d3fa9ff
Exception code: 0xe0434352
Fault offset: 0x000000000000b87d
Faulting process id: 0x95c
Faulting application start time: 0x01d56a16c9f87312
Faulting application path: C:\Program Files\Altaro\Altaro Physical Server Backup\Altaro.PhysicalBackup.Service.exe
Faulting module path: C:\Windows\system32\KERNELBASE.dll
Report Id: 4009f7d0-d60a-11e9-9565-e61f13c36143

That's exactly what I did yesterday afternoon and scheduled the box to reboot during the night.  This morning I was able to get into the console but after one click the console would lose connection to any backend data sources, giving errors such as "Unable to retrieve backup data".  I then re-installed Altaro PSB one more time, and all is well.  Looks like you were right, something was wrong with the .net framework.  Microsoft's .net repair tool solved the underlying issue.

Thanks for your help!