Fatal error on some pages after cancelling WPSTAGING cloning process

Yesterday I was cloning my WordPress website with WPSTAGING (https://de.wordpress.org/plugins/wp-staging/). Accidentally during the cloning process I closed the browser tab within which the cloning process took place, so I did NOT properly clicked the “CANCEL” button.

Today I have noticed the following problems:

• When I click on some pages within my website I get the following error: Fatal error: Unknown: Cannot use output buffering in output buffering display handlers in Unknown on line 0 It is a white page with just this error message.

• When I try to re-install WordPress I get the following message:
  Another Update is currently in progress.

• When I try to restore a backup with Updraft Plus, after clicking the restore button I immediately get the message that the restore process got cancelled. Also simply downloading a backup file from within the Updraft admin area is not working.

I don’t know what to do, I am desperate. Could you please help me how I can fix this problem that most probably arose from cancelling the WPSTAGING cloning process?