When Plesk is upgraded either from 17.5 or 17.8 with the selected Plesk 12 theme to Obsidian, this theme is preserved after the upgrade, but the layout is completely broken.
It is a Plesk bug with ID #PPP-45324. It will be fixed in the next product updates.
Until a fix becomes available, use the following workaround:
- Log into Plesk;
- Go to Tools & Settings > Schemes and Color themes and change to Onyx or Obsidian theme.