Hi,
this post is meant as information for everyone who has the same problem.
I had this bug also in 19.x and now in 20.7:
When the installer reaches "Select Task" and an option ist choosen, the installer hangs.
The solution is to CTRL-C before an option is choosen, the login again as "installer". Then the installer would run.
As I readin the forum, some other users had this problem in combination with Hyper-V. I am running OPNsense on a bare metal machine (HPE DL20 Gen10).
Greetings!