SVN server on DIR-100 after firmware upgrade inaccessible

Ok, I realize this question is fairly specific and far fetched. but there may be a more general applicable solution not specific to my setup. I'm not a networking expert but I'm hardly unknowledgable either (considering I'm a programmer and MSc. CS student); feel free asking more advanced follow up questions and I'll study it as needed...

So:

I'm running a VisualSVN server at port 8443 on a computer connected to LAN with a D-Link DIR-100 router. Prior to upgrading to latest firmware I could access this server from the Internet; after the upgrade I can't. I can still access it from LAN as before.

I've double checked all settings (port forwarding, DMZ etc.), and tried different server settings altogether: Different ports, HTTP instead of HTTPS protocol, etc.

It might specifically be some problem with how the router interfaces with SVN. For those of you who know VisualSVN, it among other things hosts a browser based page - I get as far as it asking for my username and password, but it is unable to display the list of repositories after that. Connecting directly with a SVN client doesn't work at all.

An obvious solution may be to downgrade the firmware, but since now my FTP server suddenly is accessible from the Internet after the upgrade I'm not sure I want to do that. (Oh the irony, the upgrade breaking one application and fixing another.)

I'm not surprised though, for as long as I've had various D-Link routers (10 years) they've been having weird non-orthogonal issues.

I realize the information above is too little to help me solve the problem by far, so once again, feel free to ask followup questions.