I'm getting a BadImageFormatException after our servers received a Windows Update. Googling only hints at .NET mismatches but I can't find any problems there. Jenkins Version is 2.263.1
Edit: Removing < supportedRuntime version="v2.x" /> from jenkins.exe.config seems to have done the trick although version="v4.0" was also in there. I leave this open in case someone has an explanation.
I upgraded from Jenkins ver. 2.222.4 to Jenkins 2.303.2 from within the web UI. New versions came up, but the service would not come up again, failing with error mentioned above.
Jenkins.exe.config was last modified 2016, which let me conclude this is a stray file from a previous version.
Remove the stray jenkins.exe.config and you're all set.