I recently ran into an issue with Enketo in ODK Central when trying to fill forms in my web browser (or edit a submission originally from Collect). Upon attempted load, Enketo threw an error saying it was 'unable to detect delimiting character, defaulting to ',' (see screenshot below). This was odd to me, as Enketo worked fine for me on my install with other, similar form definitions, and Collect handled this specific form perfectly.
I was unable to find much information about this online, so did some troubleshooting by steadily removing things from my form definition until it behaved. It turned out this was an issue caused by some of my csv attachments to the form only containing one column of data. When I added a second column to the csv in question with junk data, this error resolved and the form was submittable through my web browser.
I'm making this post here so that other people encountering this problem in the future can hopefully find a solution to their problem faster than I did!
Original error message: