I came across a very vexing issue. When I tried to change the metadata on a number of my files, I would get the error message "cannot be opened for writing". I see from a number of posts that others have been having the same issue, but I couldn't find a satisfactory solution in the forums.
This was particularly frustrating because other Metadata editors didn't seem to have a problem opening the file and saving the data. For example, Tag&Rename had no issue and neither did the editor built into WinAmp. Why use MP3tag, you are probably asking by now??!
I had a problem with my server, Serviio, in that it would not read the information in the "Album Artist" field of many of my rips. I discovered that by creating an Action in MP3tag and moving the info from "Album Artist" to "Albumartist", it solved the problem.
This worked for most of my files, but not for a significant number. I would note that the ones it didn't work on were all rips from vinyl using Audacity.
Interestingly, most of my vinyl rips were OK but a significant number were not.
To solve the problem, I reloaded the file in Audacity and re-saved. Without fail, the "cannot be opened for writing" problem disappeared. Quite a lot of work, but it solved the problem.
I'm still puzzled though. I assume that the file had some issues but no other editor that I used seemed to have a problem with the file. And re-saves using the same program I originally created the file in never once created an unreadable file.