Two problems: Can't change column widths, can't move actions to other computer.

On one machine, I can't change the widths of columns at all. Has anyone seen this before.

Actions I've made on that machine do not work when I copy the AppData "actions" folder to the proper place on the other machine. All of the .mta files are there. They will not appear in the actions menu. Has anyone got a solution for this?

If changing the column width on one machine and it does not on the other, it would be worthwhile to investigate the differences between the 2 computers. It is very difficult from the distance to tell what the cause may be.
For me it is not clear if Windows does not allow to modify the column width or whether this information is not stored.

On the "actions" topic: You have find out whether MP3tag starts as a local installation or some kind of portable one. In the latter case, the data folders reside inside the program folder.
You can check this when you look at the modifcation date of the file mp3tag.cfg: It should be somewhere near the last time you used it. This would then be the folder that is read by MP3tag.

It's not a question of storing column widths. It won't let me use the mouse to alter the widths at all. I've installed MP3Tag on numerous Windows 7 and Windows 10 machines and never seen this behavior before.

Neither installation is portable. The actions that I make on one machine don't even appear on the menu of the machine i move them to. based on your suggestion, I checked and made a new action on that second machine. The action is saved but not in AppData>Roaming>Mp3Tag>Data>Actions.

So the mystery is where is this instance of MP3Tag storing actions?

Well, that installation stores actions in Program Files (86)>Mp3Tag>Data>Actions

I don't know why, but mystery solved.

And after a reboot, I can resize columns. All is well.