Thanks for the info. I have uninstalled the plugins that can be uninstalled.
I think the cb.wall also belongs to the awdwall extension, but as it didn't work anymore I have removed it too.

As for the language plugin "Dutch", how do I update it, or where do I find a new version?
I already installed nl-NL, which is the same as Dutch, so I thought that I had already updated it that way.

As for the language plugin "CB Paid Subscriptions Dutch", where do I find a new version?

As for the DOCman integration:
My users have the ability to upgrade to a paid account, at which point they are supposed to be given access to a certain DOCMan category (until their paid upgrade expires). If DOCMan integration is no longer supported for CBSubs, is there another way I can still make this an automatic process? Or is there another extension that is similar to DOCMan and which CBSubs does support integration for?

Thanks for the links, I found the languages I need and installed them successfully.

Good news about DOCMan now organizing access like that, it does make it easier and it does indeed make a CBSubs DOCMan integration no longer needed. That reminds me I'll have to update DOCMan to the latest version too.

Getting back to the strange login problem with newly registered users:

I made an exact copy of the website in a different subdomain, with an exact copy of the MySQL database.

When I try to register an account there, using a password with numbers, I am able to login without a problem. The only thing that is different is that the live site has an SSL-certificate enabled on the domain, and the copy does not. But when I disable SSL in Joomla, the problem still persists.

I even made sure that the redirect URL after the first login was set up correctly, and redirect URL's in the login module as well. But it makes no difference.

The encryption login form setting was already set to "no" in the module, and live_site was already empty. I have added the non-SEF URL as the login (starting from index.php in the modules and the complete URL, including https, in the first login redirect in the CB settings), but it makes no difference.