scheduled refresh COM error: mscorlib, An element with the same key has already been added

Anonymous there is a workaround posted by MS at the bottom of this thread: https://community.powerbi.com/t5/Report-Server/December-2020-Update-Issues-Regarding-Power-BI-Report-Server/td-p/1560294

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How to Disable Enhanced Dataset Metadata

Note that Desktop authors can control if the enhanced metadata format is used when editing reports. If you are experiencing the above or similar errors, we recommend disabling the options in desktop as follows:

1. Turn off the Enhanced Dataset Metadata option in Power BI Desktop RS
   1.  In Options, under Preview Features, uncheck “Store datasets using enhanced metadata format”
2. Restart Power BI Desktop for Report Server
3. Restore the old report from the hidden backup in AppData.
   1. Before the file is upgraded to the new format, a backup copy is saved in the app data folder. For example: C:\Users\myuser\AppData\Local\Microsoft\Power BI Desktop SSRS\TempSaves\Backups.
   2. Open the backup and you should be able to save the old version of the report back to the server.

The automatic refresh works again for me after restoring the backed-up version.