Hello Nuengrudi,

I've checked your site and didn't see any error according to screenshot you shared.
Can you please tell me steps to replicate this issue, so i'll be able to check it.

Dear support

Pls, check in my backend of joomla on administraton. in Main Menu and Module still an error.

Thank you

Hello Nuengrudi,

I've checked your site and able to replicate this issue. I serached for this error and found that this issue is coming because some of files missing. When you upgrade to joomla 3.8 from previous version it's not updated properly.
See below mentioned link for more info.
https://issues.joomla.org/tracker/joomla-cms/18003
https://github.com/joomla/joomla-cms/issues/18023
https://github.com/joomla/joomla-cms/issues/18003

This issue is coming after installing payplans because PayPlans Menu Access and Payplans Module Access app's are enabled. For now to resolve this issue i've disabled these plugins. I would like to suggest you to re-install joomla 3.8.11 package again to resolve this issue as per above mentioned solutions.

Update me if you have any query.