Hey @sotiri and @thomas_pronk! I have an update for you! I think I figured out what the problem is. There seems to be an issue with the piloting mode and closing sessions of my experiment on iPad Safari or Firefox iOS.
While I am in PILOTING mode, the experiment complains at the end with the closing session error as mentioned above multiple times. This is propably due to the logging error with log4javascript.js which you mentioned (?).
BUT once I change the mode to RUNNING, the experiment runs perfectly fine on both iPad browsers Safari and Firefox iOS! There is no longer a problem with closing the session propably and I am able to download the results afterwards without problem.
I don’t know what this sorcery is, but for whatever reason, it seems to work for now and I hope it stays that way 
That is probably something you or someone of the developer team should look into for future updates though, right? 