14.0.4 -> 15.0.4 Updater stuck at step 3 - Server gives AH01630 error

Hi there,

tried to Upgrade via Updater from 14.0.4 to 15.0.4.

Updater App got stuck. Restarting Updater results in white page with
“Step 3 is currently in process. Please reload this page later.”

The server log says:
AH01630: client denied by server configuration: path-to-.ocdata

My Hoster is Strato. Thankful for any help/advice,