It's a curious error message. It *READS* like you are trying to write data to Excel, not just read from it.
First thing to try is to just *STOP* doing any WRITEs to the SQL Server DB, at all. Just do reads from Excel and see if the problem still persists.
And if it does, then for starters, I'd try narrowing down just *which* field is causing the problem.
But I don't see how we can help you debug code when we can't see the code. So if you don't figure it out, maybe you should post the code? At least the seemingly relevant parts?