Yes, it really looks like some configuration files are causing this issue due to unsuccessful upgrade.
Thus, the best next thing is to repeat the upgrade but with a new version this time. You can follow the same installation instruction (link here) and upgrade to Sitefinity 5.1 instead. The thing is that after Sitefinity 5.0, we have introduced separate assemblies for the different modules and sometimes this may cause upgrade complications. Other that it is a fully supported upgrade scenario, so please let me know how it will go.
As for the version, I usually pick the latest version (with the biggest number) as it is considered to be better than the previous one.
Please try to upgrade one more time and let me know on the outcome.
P.S. If this doesn't work, we may request sending your project to us, so please keep us posted on the result.
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