The history: I hung onto my old pc running Win7 (and thus unable to update Calibre for a long time) until 2022/23 when I upgraded to Win10. I still did not bother to update Calibre as everything worked great ... until troubleshooting problems that were popping up as my computer failed.
I had downloaded several programs during troubleshooting that I ultimately deleted but problems with some of my old software, like Adobe Acrobat X, showed up. The "go back to an earlier date" function did not work and a MS support agent, via remote desktop, did a reinstall of Win10, keeping all my files, etc. taking me further down the rabbit hole.
I routinely read books via the View button in Calibre and so was surprised when I discovered that was no longer working. Since I had not done a Calibre update for eons because of running Win7, I may have just updated it before I lost this function. To my surprise, however, ADE4.5 would not open the epubs, either. Adobe 3.0, however, still worked and contained most of the epubs I had borrowed over a couple of decades from public libraries where I had lived.
My computer failed and I replaced it with a "new to me" PC running Win11. I had hoped this would resolve the issue with being unable to read epubs but it actually made it worse. I did not think to import the library that Adobe 3.0 had created and now Adobe 3.0 will not open epubs.
It does not matter what program I choose as the default to open epubs. None will. Essentially my entire Calibre library is toast (and as an 80 year old who spends more time reading than anything else, it is extensive).
The only way I am able to read epubs is via Calibre's book edit in the preview screen.
I read that an old plugin, DeDRM, might be the problem and found a newer version via github: DeDRM_tools_10.0.3. I've installed it and am still unable to read epubs. I don't know if there are additional settings that one should change to make it work. The library where I live now is tiny and does not offer ebooks so I don't know if that is how I got the Adobe readers to work years ago and why they don't now.
I apologize for this lengthy account, but I have no way of knowing, what, if any, of what I've described is pertinent and might be helpful. I do hope that someone can help me recover the ability to read epubs.