I've run a bunch of tests and the only way I can duplicate the problem is if a poll is deleted from a form thread but still exists in the form promotion system.
There will be a trap for this in the next release, so once that's out you'll get a PC from the form system telling you which thread is causing the problem. It will also wipe the poll from the form promotion system when that PC is sent so it won't happen again.
If you want to manually do this now it's a bit complicated, but can be done using PHPMyAdmin.
In PHPMyAdmin access the xf_poll table.
In PHPMyAdmin access the xf_snog_forms_promotions table in a NEW WINDOW.
Check that each promotion "poll_id" in xf_snog_forms_promotions exists in the xf_poll table. If it doesn't exist, delete it from the xf_snog_forms_promotions table.
IGNORE any entry where the poll_id equals 0 (zero) in the xf_snog_forms_promotions table. Those are polls waiting for manual approval using the approve/deny links in the thread.
As I've said before...
https://xenforo.com/community/threads/advanced-forms-paid.133580/page-20#post-1299354
The next release of the forms system will be when XenForo 2.0.12 is made because the next release of the forms system relies on changes being made in that version of XenForo itself.