I had a 2TB logical volume, which was almost full. So I extended the volume to 2.5TB in the GUI. That worked fine. However, when I tried to extend the file system through the GUI, I got an error.
I tried doing the same from the shell, and I got an error as well: "resize2fs: Operation not permitted While trying to add group #16384"
After some Googling, it turned out that the problem was that my journal became too small. I solved it by rebooting in single user mode, removing the journal and recreating it (commands are found here: http://blog.rbrt.com/2009/07/r…oken-your-journal-is.html). It went from 64M to 128M. After that, a resize went fine.
I'm not sure why this issue occured. Either it is by design and I ran into a limit. Or it is caused by the way OMV creates its filesystems (with a journal file that is too small).
Posting this in case someone runs into the same issue, and maybe for a dev to have a look at it.