This problem may be a result when you have a razor page in mvc with a model that has some validation rules. When you post from a form and you forget to display validation errors on some field, then this message might come up. Speculation: this could be if the method you are posting to is different and used by other sources or resides in a different place than the method serving the original request.
So because it's different, it can't return to the original page to display or handle the errors because the excecution and model state is not the same (something like that).
It can be slightly difficult to discover, but easy mistake to do. Make sure your recieving method actually validates all possible ways to post to it.
for instance, even if you have serverside validation that actually makes it impossible to write in the form a string that is bigger than the max allowed by your validation, there could be other ways and sources that post to the recieving method.