Thomas, many have experienced this problem. Here are the instructions how to fix it.
First make hidden files visible in Windows folder options. Close WT library.
1- Go to C:\ProgramData\Watchtower\WTLibrary (This is a hidden file; you can only see it if hidden files are made visible)
2-- delete the two files, "ca-bundle.zip" and "cert.pem"
3- Open Watchtower Library, go to help/check for updates and try again.
This has fixed the problem for many people.
Kind regards
Chris