500 Internal Server Error after attempting to switch from subdirectory to addon domain

directory site that was hosted at https://example.com/mysubdir

Now, I purchased a new domain and added it to my shared hosting account as an add-on domain.

To change the domain settings, I did the following:

1. Pointed the domain towards the sub-directory (successfully)
2. Got into the database and updated site URL and site Home to the new domain
3. Updated site permalinks using Better Search Replace

What’s happening is that I can successfully access the home page but all other pages return 500 Internal Server Error. The back-end is though accessible and I can see and edit all pages. But every time I try to view a page with the exception of the home page, I get the 500 Internal Server Error.

I asked my web host about it, and got a link to an article, where they described the process – and it seems to be exactly as I did it with the exception of rewriting the permalinks. But rewriting the permalinks IS necessary.

Can you please help me figure this out?

Just FYI I am using SSL on both domains, that is the sub-directory uses one custom SSL installed at the primary domain and the add-on domain uses another custom SSL. Could that mess things up?

An update & resolution:
I asked the web host for help and they found that I had a piece of code in .htaccess limiting the access to wp-config.php. Once the code was removed, all worked as it should.