Your next step now that you know you don't have a hardware/software or hosting problem is to do the NOF shuffle (the full 3 step export routine then import under a different name). See if this resolves the not validating problem after you do a full site generation and upload all pages. You want to make sure you upload all pages from the new site project to remove the possibility that any orphans might be causing your issue
If the form still does not validate after "the shuffle" and full site upload, you need to create a new form page within your site project and see if this solves the problem. If it still fails validation you need to look at other externally inserted coding within your site project for conflicts.
You can do all of this on a test folder on the domain so that you do not disrupt any live content.