SSRS Report Rendering error in M-Files

I don't think it's related to M-Files. In my previous experience with SSPRS, it was actually related to XML format that is constantly changing in every version of SSRS. Get your SSRS version (you got this one) and update the XML tags in the RDL file accordingly. You can find correct definitions and format in stackoverflow or even ChatGPT and likes AI should be able to provide correct definitions.

I tried this and changed the schema to 2010's version and with chatgpt adjusted everything so it would upload to the SSRS server and sadly got the error again

Is it the same error or different one ?

Yes same error. It seems no matter what I do I get this error. Even if i make a report with just the word test on it. It worked once but then it stopped working again with that test report after the first time

We have picked up on the cause of the issue. There is a difference between the URL structure of working reports and the report breaking. If you use the previous URL structure then M-Files will read it and render the report fine. However, if you put the old URL in a browser it will auto convert to the new URL structure. 

For Example we had a report Test in folder TestFolder the old URL would be:
ServerName/ReportServer?/TestFolder/Test 

The browser would display the URL as the below: 
ServerName/ReportServer/Pages/ReportViewer.aspx?/TestFolder/Test.

This is the URL we used on the report object in M-Files, which would give the mentioned error if trying to render the report in the app, clicking on the URL in the metadata would render it in the browser fine, causing our confusion.

Weirdly, M-Files prefers the previous URL structure. Now we know.

We have picked up on the cause of the issue. There is a difference between the URL structure of working reports and the report breaking. If you use the previous URL structure then M-Files will read it and render the report fine. However, if you put the old URL in a browser it will auto convert to the new URL structure. 

For Example we had a report Test in folder TestFolder the old URL would be:
ServerName/ReportServer?/TestFolder/Test 

The browser would display the URL as the below: 
ServerName/ReportServer/Pages/ReportViewer.aspx?/TestFolder/Test.

This is the URL we used on the report object in M-Files, which would give the mentioned error if trying to render the report in the app, clicking on the URL in the metadata would render it in the browser fine, causing our confusion.

Weirdly, M-Files prefers the previous URL structure. Now we know.