Having an issue uploading pdf files and photos to some websites

I’ve been struggling with this rather weird issue for months and am so flummoxed I don’t really know where to start to try to fix it. Here’s what I’ve got:
-If I try to upload a picture or a pdf to ravelry (an online fiber arts community) from my mac mini the upload hangs.
-After that I can no longer get any part of ravelry and some other websites to load but I can still download email and some other websites are fine.
-I can fix this by rebooting my router.
-It’s not just ravelry, I tried to upload to a pdf hosting site and had the same problem, but I uploaded a very large pdf to a printer last night with no problems.
-If I put the file on a thumb drive and use my husband’s computer (windows machine) it works fine.
-If I transfer the photo to my iPad I can upload it to Ravelry just fine.

So I’m guessing there is some bad interaction between how ravelry (and other websites) handle data and how my computer is trying to handle data. But that’s just a guess and since I don’t know any of the technical aspects of how any of this works I don’t even have the vocabulary to go looking for an answer.

Any ideas?