This error doesn't have anything to do with upgrading your Mendix version persé; rather, you have a select page which is not structured correctly. Double click the error to navigate to the page in error. I suggest rebuilding it as the error says – you can only have a single list view, data grid, or template grid in a standard Mendix select page. If you want to do something different you will need to simulate your own select logic via a combination of microflows and a select page.
The error seems clear and Querijn’s answer is the first that comes to mind, but as you state correctly it is started after the upgrade. We need to know a bit more to help out. Please take some steps to come to the cause of the error:
- find out which select page is causing the error (hopefully just one)
- if you remove it, does the error go away?
- when does the error occur, during startup, or at specific steps.