Thanks ShellShock, for spotting yet another problem, for which there seems to be no explanation and no solution either, except for trying another program. A program to which I am not familiar, but I will certainly give it a go in the coming days/weeks.
After the problems experience with the (error message No module named PyQt4), then not being able to modify the IOS Reader Plug in, I thought we were almost there...just lacking a few buttons/connections... Not so!

Now, every time I start Calibre, I have to update the IOS Reader Plug in again, and I (sometimes) get weird error messages which I just accept since there is nothing else I can do (yes, this is a new issue since this week!).
But once all that is done and Calibre seems to be working as it should, I am for now happy because I can make my iPad communicate with my Calibre library via the Marvin DropBox connector, so at least I can keep on reading. I have spent weeks and weeks getting that Calibre library set up, so I am not ready to abandon!
I so hope that a solution is found to all the problems reported just about everywhere on this forum, especially related to (so it seems) IOS 8 upgraders; it would be so nice if we could all go back to being happy Marvin/Calibre users
Good luck and thank you for everyone involved in making this happen!