My guess is your antivirus has changed. Try disabling it.

I don't have a 3rd party antivirus loaded, I'm just using Windows built-in security.

Still, do you think that could be it?

Users with gmx have lately been reporting problems. You might want to contact them.

I have sent them a note. Their first response was a generic "check your settings" kind of thing, which as I had explained to them in my note, I had already done. I replied back and still haven't received a response. I doubt that I will, they're not very responsive.

I am in the USA and cannot connect to gmx with thunderbird unless I use a vpn to show I'm from somewhere else like France. EDIT: I used my vpn to show I'm somewhere else in the usa but close to where I am. I had success connecting to gmx. So, my question at this point is... I am using t-mobile for my internet, could they be the culprit?

Yep, me too. I discovered that if I use a VPN that routes my computer through another country, Thunderbird will update my GMX accounts. I'm currently using a city in Canada.

I wonder why GMX is excluding the USA when using Thunderbird? Especially since I can sync with GMX when I use the "new" Outlook...