Hello @Holger Gerling ,
Thank you for your post! I spoke to my team and based off your screenshots and investigation I did with our Azure Support Team, we think it's best for you to work more closely with our Support Engineers to get this issue resolved.
Please run the data collection tool on your OMS Gateway computer found here and per the article, choose the Agent not reporting data or heartbeat data missing option under the Scenario Covered section of this article. Then once this the diagnostic report has completed, there will be a zip file which Azure Support can help you troubleshoot.
If you already have a support plan then to find the root cause of the issue here, I recommend you reach the Azure technical support team by raising a support ticket as mentioned here.
Otherwise, if you don't have a support plan then to find the root cause of the issue here, please send an email with subject "Attn:Carlos" to AzCommunity@microsoft.com with your subscription ID and a link of this question for context and we will assist you with engaging Azure technical support for a free technical support request.
Thank you again for your time and patience throughout this issue.
Carlos V.