Hello Oliver S,
I have installed the latest Notes client v11.0.1FP1 under Mac OS X Catalina 10.15.5 and it is stable without any issues till now :). So I didn't face this issue but if you face the same then you can raise a support case.
But make sure to collect the NSD logs and Console.log and upload it in the case:-
For collecting logs you can refer the below KB:-
https://support.hcltechsw.com/csm?id=kb_article&sysparm_article=KB0076396
Thanks!!