if i installed Nextcloud behind a Nginx Reverse Proxy:
a) is it necessary to create a seperate SSL Certificate for Nextcloud or is Reverse Proxy SSL certificate sufficient?
b) shall the internal connection between ReverseProxy and Nextcloud instance SSL secured ?
A) Just apply the SSL to one or the other NOT both.
B) This depends on where your Reverse Proxy and NC are, if they are on the same lan, NO, if you are connecting the Reverse Proxy over the internet then YES.
I would do the SSL on the NC instance, that solves all your problems.