This sounds like a problem that dogged me for months over last Christmas when I could not post a picture to my own forum (the same Invision Power as this) getting that -200 error. Other people could post a picture and I had a couple of very frustrating months with the Forum software creators blaming my web space providers and the web space providers blaming Invision Power. There are all sorts of permissions at web level to control what files can be accessed by the public and which can't. Lists of them in fact
Other issues I had with emails led me to form my own opinions, because all the technical people gave up and said there was nothing they could do. I have no proof of this, but I think my issue came about, because I was posting a picture in Australia to a web site located elsewhere in the world and it was obviously my location that was the issue. While that should not have been a problem it was.
In the end, I was forced to move my forum to an Australian server away from the other one in the USA and it immediately started working perfectly and has done so ever since.
So, I hope I am wrong and you don't have the same issues as me, because I couldn't get any technical people to help, mainly because they could not reproduce the fault. If you have a friend close by. Try taking what you need there and uploading from another location, you may find it works.That will not be a solution, but it may indicate that your problem is the same or similar to mine
During my detective work, I bought a new modem, because the technical guys suggested it, but it didn't change the -200 fault. It did improve my wireless through the house so at least my $200 got me something. I suspected it was my ISP at fault, but friends and family also in Australia, were able to post a picture on my forum using the same ISP
I think it has something to do with that handshake that two computers have to make before transferring files. If its not made quick enough it fails.
Best of luck