For anyone reading this thread and having the same issue. I have been using Calibre since v2, I believe. I enjoyed it a lot. But my libraries started growing so I moved them to external HDDs. But then I started using more laptops. And if I wanted to access the libs on another laptop, I had to manually eject each HDD (there were 10 HDDs at one point) in order to access them on the other laptop. That takes a lot of time.
I had tons of issues incl. multiple library corruptions. Which took days if not weeks to solve / repair.
In the end I created a NAS from an old laptop using OpenMediaVault . The libs were still on the external HDDs, but this time they were connected to the old laptop and running on ext4 extension. The intention was to use Calibre docker on the server and access the libraries like that. Which I actually did. And I still do. But I realised it is much easier to actually access the libs via SMB. Meaning I access the lib folders just like any other folder and thus Calibre does the same. I also added Tailscale to access my libs from another location. And the server is always on. My work PC is not.
It all worked like a charm. No issues at all. But then I created a new server in a desktop PC with internal HDDs. Because it is much more stable, of course. I moved my libs to the new server. And this is when I started having this issue with the caltrash.
In the end I realized the problem is easily fixable by just deleting the caltrash folder and then trying to access the lib again. Worked like magic. So definitely I will do this with the other libs I might have issue with. I do believe it has something to do with permissions from the server side as I realized I also don't have access to the recycle folder on those HDDs which I had on the old server. So I will have to look into that and it might solve this tiny issue with caltrash.
So as a conclusion, I just asked for a solution to the caltrash problem. And almost all of you just diverged by letting me know that Calibre does not work in a server. Well, it works. It's been working for for me for almost 2 years now. And I have fewer and less problematic issues since I moved my libs on the server.
Have a nice day!