Short answer: As long as primary ownership of your current Maps listing remains with the former employee, you won't be able to fix the hours permanently; any external edits will be put on hold or rejected by the authorized account. The quickest route is to go to the existing profile using the shop's official Google account and request a formal transfer of ownership via the 'Claim this business' option.
First, search for your business on Google Maps and click the 'Claim this business' or 'Own this business?' link on the profile details. The system will display a masked version of the current owner's email address (like a***@gmail.com). At this point, select 'Request access', pick 'Ownership' as the access level, fill out the form confirming you are the business owner, and submit it.
This request will be automatically forwarded to the former employee's registered email, giving them a 3-day window to respond. If the ex-employee approves, control transfers directly to you. If they don't respond at all within 3 days a 'Verify' or 'Claim ownership' button will activate on your Google dashboard. You can then claim ownership unilaterally by requesting a verification code by mail or verifying the business via phone or video.
Once you take over ownership head to 'Users' under Business Profile Settings. Completely remove the permissions of the former employee and any other accounts you don't recognize. Next, update and save your regular schedule under 'Hours', and specifically add Germany's public holidays (Feiertage) under 'Special hours'. That way your profile will automatically show as closed on holidays.