OK, I got it fixed, but don't ask me how. LOL.
I had already tried uninstalling then re installing version 2.91, but the first time I tried it, it didn't help. The same problem persisted.
So, I went to the Winamp Heaven web site and got a previous version of Winamp, version 2.51. Then I uninstalled my current version and installed this one instead. And lo and behold, now the disk writer plugin worked.
Then I saved the disk writer plugin file to another location with the intention of trying to use it with version 2.91.
Then I uninstalled version 2.51 and re installed version 2.91. I was going to take the disk writer plugin that came with version 2.51 and try to use it with version 2.91. However, that proved unnecessary, because for some reason not known to me the disk writer plugin that came with version 2.91 now works just fine. So I didn't need to use the older version of the plugin after all.
I don't know why my previous attempt at uninstalling then re installing didn't work, but I guess that all is well that ends well.
BTW, the mp3 Pro plugin was not the culprit. I still have it and there is no problem now.