www.makemkv.com

I tried to create an account using my standard email address (ending in @proton.me), but the site wouldn't accept it. However, I was able to use a random temporary-email-provider. Afer I created this account I tried changing it in the User Control Panel, but had no luck either, which is unfortunate since I will eventually lose access to the temporary email.
Is Proton-Mail banned, or did I do something wrong?

Yes, proton.me is one of several domains blocked for past spam activity. Quite a few proton.me users instantly started posting spam messages, to the point where the domain was finally blocked.

It is unfortunate for the people who find a legitimate reason to use them, but you're actually the first person to ask about it since the domain was banned several years ago.

Unlucky. Thanks for the response, though.

is gmail not allowed? I have made a few post recently that I didn't get reason for not being posted or they got posted.

You are required to have a minimum number of posts on the forum to "post freely", due to the spam we get. Me being sick for 12 days screwed that up. I have made an inquiry of the author to deal with that.