@girish Many thanks Girish. It must have been a space problem indeed. Looking at the log, the processed froze while downloading the last big part of the image (~570MB). I tried to duplicate the app, which worked fine but and the process also froze when trying to update it (also while downloading that part the image).
Anyway, I had a about 3GB of free space which I though would be enough to update Wekan but after doing some cleaning up on the server (like deleting old apps I wasn't using), I went to about 7GB of free space and the update started working again.