Being part of a small business, we used to use the HMRC provided Basic PAYE Tools (BPT) for Payroll.  With the changes in this tax year a new version has been provided to support the RTI submissions.
The old software front end was built in Flash with a Python/MySQL backend.  Whether or not that was a good idea is debateable but at least it was clear.  The new version appears to be Python with SQLlite utilising a small HTTP server running on the local machine.
The new version now looks more like the web site and (IMHO) is very poorly designed.  Its over complicated and gives very little feedback.
We had problems submitting any return.  A standard ‘page cannot be displayed’ was shown.  Very helpful.
The HMRC helpdesks we either engaged or no help so I did some digging……
I finally stumbled on the solution.  We had imported all of our data from the 2012-2013 BPT system, company details and all.  Looking at an employees records, all looked well.  However, after clicking ‘edit PAYE details’ some fields were shown to be mandatory.  Once these we filled in the preview and submit all worked !!!!
There does not seem to be any real way to submit any feedback to HMRC but I may try an alternate method …… 🙂