Can I somehow assign a reminder date (or even better also a time, if needed) so that DirOpus will remind me with a popup or with a tab that lists the reminder files that I have to deal with the file (for example invoice?)
Or is it possible to create a custom column "due" and search / filter for that somehow?
For pop-up reminders, you're best off using a reminder tool, as that isn't something Opus is designed around.
If the tool lets you create reminders via a command line or scripting interface, you could have a button or context menu in Opus which tells it to create a reminder for a given file.
Could be done using scripting. Probably not a huge amount of script code would be needed, but not a trivial amount either.
I see. Thanks. XYPlorer has custom columns out of the box, so I was very confident that such a thing is available in DirOpus, too.
Please implement this for the next version 
I am sure that having a kind of date property for a document is not such an exotic requirement.
Opus has custom columns as well, implemented by script code (which only needs to be complex if the thing it's doing is complex).
If you just want a free-form text field that you can apply to files, Opus also already has that in the form of user comments/descriptions.
I use comments in many cases. But I would need a date column that I can use for sorting, if needed.
If anybody knows a tool that can assign a due date to a file and show a popup when it is due, then I would be happy to get a hint on that.