Well it turns out it wasn't a port at all but my antivirus, despite being disabled, was still blocking the download. I fixed the problem and it just finished its download. Thank you so much for your help Robert I really appreciate it

Apologies for bringing up an old thread, I didn't look to see if this was ever solved elsewhere.

I have a fix for a very similar problem: my game update was stuck at 4%, not displaying any download speed, and then I got the popup that said a required file was in use, and I couldn't proceed further.

What I did to get past this was (on Windows 7) open the task manager and looked for all processes related to ESO. Anything that said it's from ZeniMax, or Bethesda, or Zo (like the crash reporting background process), I killed. Then I started the launcher again, and it took a few moments, but the update was finally able to proceed.

Just wanted to leave this here in case there are any other people stuck like this!