Description of my issue:
I successfully upgraded the website from J3.7.2 to J3.8.5.
Took a backup using Akeeba Backup Pro ver 5.4.0 (successful).
Upgraded to Akeeba Backup Pro ver 6.0.0 and tried to backup (backup does not go beyond the initialisation stage).
Installed the previous version (Akeeba Backup Pro ver 5.4.0) and tried to take a backup (backup does not go beyond the initialisation stage now). None of the versions below ver 6.0.0 work after this.
I restored the site back to the point where Joomla is version 3.8.5 and Akeeba is 5.4.0. Backup works with this combination. Upgraded Akeeba to the latest 6.0.0 and I have the backup issue again. Besides this issue, the site seems to function properly.
I read on one of the tickets about selecting "Use IFRAMEs instead of AJAX". This made no difference.
What could be the issue?