iMac 2020 with iMac 2012 as 2. Monitor not working after upgrade to Big Sur

After upgrading my iMac 2020 to Big Sur, my iMac 2012 (Catalina) will not be recognized as 2. Monitor.

"Recognizing Monitors" is also not working.

I´m using the Original Apple Thunderbolt 3 to Thunderbolt 2 - Adapter with Thunderbolt 2 cable.

In the "Systemreport", the connected Cable ist listed.
iMac 2020 with iMac 2012 as 2. Monitor not working after upgrade to Big Sur
 
 
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