problems with change of domain

I had intitially configured my primary domain to point to a sub-folder (myprimarydomain.com/blog) where I had installed my WP files
This worked fine. Then I decided to change the domain name used for my blog (blogdomain.com) that I pointed to the same sub-folder (primarydomain.com/blog)
Since I have several problems:

1st: I cannot change my permalinks to custom (it appears always the same message: you must update the .htaccess file, which I change but it still does not work.

2nd: I tried to add a password to my blog site using the .htaccess file, but then I had a message

“A misconfiguration on the server caused a hiccup.
Check the server logs, fix the problem, then try again.”

which is a preconfigured error from the file 500.php
I checked the log errors but I could not find something

I think the origin of all the problems is the change of the primary domain to the blogdomain but I do not know what and how to fix it.
many thanks in advance any help/idea