Hey Everyone,
Bit late to this thread but there's not much else I could find relevant to this issue, so thought it worth a post.
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We have a script that creates one map per job, on average this is 5 jobs per user at a time. Offline maps are set to create sync versions per 'user' though (i.e. multiple maps but only one sync version per user). Note: all of these maps are almost identical except for their map name, map extent and bookmarked work area.
I found that out of 6 field users, only 2 users had this error appearing. Also once this error appeared for one map, it seemed that it would then appear when syncing any one of their 5 job maps. I used the suggestion Carlos Porras and Danielle Kulas had been discussing, to re-enable editing on un-editable layers. I only went into one of the 5 maps and re-enabled editing on the one reference item we have. Then to my surprise this not only allowed this map to sync but seemed to have a roll on affect and all maps were then syncable for that user.
I also agree strongly with Carlos Porras's comment "try to sync on a good wifi connection". We have seen a lot of evidence that proves that syncing on a weaker WiFi connection will have a far higher chance of causing syncing issues.
TL;DR: I re-enabled editing on the only un-editable layer in the webmap, then was able to sync.
Thanks!
Hamish