Hi,
I am linking the only sollution that worked for me in a this situation when upgrading from 18.05 to 18.06 and I got stuck af step 5:
Credits go to @rahlquist
In short (my php version was 7.3)
/etc/php/7.3/cli/php.ini had -1 for memory limit, and it should stay that way
Returning the setting back to -1 this error went away.
https://help.nextclhttps://help.nextcloud.com/t/stuck-on-update-process-step-5-is-currently-in-process-please-reload-this-page-later/58371/6oud.com/t/allowed-memory-size-of-xxx-bytes-exhausted/39371/11