Looking in the output it does not show a build error.
If the application is running and you save, the changes are reflected in the running application. I can also stop the application and run it (build is successful). Design still shows the error. If I restart Radzen Blazor Studio, reopen the project and reopen a page in design it does show the page.
I am not sure I understand. Does the problem happen every time you open this app? Does it happen in new apps? Please provide as many details as you think could be relevant.
I don't think we've made any changes related to design-time build in 1.50.0. Can you try installing an earlier version? I do suspect OneDrive on this one as we have seen process locking issues with it since forever. It constantly tries to sync files while a build is running.
I will try an earlier build it even happens with OneDrive closed (not running, not syncing). I've not had this issue previously and can't see a pattern.