In case this trips anyone else in future:
In the repeat_count field in the example file @mathieubossaert has attached, the count is eg ${numpoints_polygon}. This will not throw an error in Enketo, and will function in Collect, but it will throw an error message on validation in Collect without pointing anywhere specific and preventing save/submit.
Sorry, form save failed! java.lang.Double cannot be cast to java.lang.Integer
Changing the repeat_count to eg int(${numpoints_polygon}) will fix this in Collect
Two posts tipped me off to this, one & two
(An old post points to the docs for a FAQ on how to get error logs, but the link is no more and redirects to docs)