Update 3: Ok, so as this was blocking everything here I just risked it, went in via ssh and truncated the nextcloud.log and nextcloud.log.1 files to zero.
Following roughly this post here: Clear logfile NOT logrotate [SOLVED]
I then could login into Plesk again and found 3 nextloud.log files, one with 116 GB still, so I guess there was a second nextcloud.log file. But deleted all 3 nextcloud.log files and it seems to have worked. Now there is a new nextcloud.log file, but it is still with 0 B, so I don’t think it is filling that quickly. Will need to monitor this.