Judging by the error message it seems that the issue occurs in the web.config file, however it is hard to tell what could have caused this issue.
Can you please compare the web.config of your upgraded project
with the web.config of the _EmptyProject
the Sitefinity version you have upgraded to and check if there are any missing entries? If yes, please add the missing entries and test the behavior again.
You may also try to replace the web.config in your project with the one from the _EmptyProject of your current Sitefinity version.
In addition to this, what I can also suggest is to restore the backup of the project before the upgrade and perform the upgrade again by strictly following the instructions from our upgrade documentation
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