ArcGIS Collector - the dreaded "Unable to Synchronize Replica" message (Importing delta data changes failed)
TL;DR - Trying to sync returns the error "Importing delta data changes failed". This can be caused if multiple layers in the web map originate from the same data source. Refer to this page for more details on this behavior. It is recommended to publish layers intended for offline editing as separate services.
I've recently been working with Esri tech support to determine why I was receiving this error message when attempting to sync edits in Collector.
Error Code '500': 'Unable to Synchronize Replica.' Importing delta data changes failed.
I went straight to tech support for this one rather than posting a question on Geonet because it was a time-sensitive issue and we needed to try to recover our offline edits and figure out the root cause of the issue as soon as possible, but I wanted to follow up here by writing a discussion, just for the slight chance that this post might pop up in a Google search one day and help someone else who is facing the same issue.
First off, yes, we attempted the workflow recommended by Esri to Access offline edits from Collector for ArcGIS directly from an Android or iOS device. This workflow has been successful for me in the past, but we were encountering access issue with the iPads that turned out to be the result of an iOS bug that could only be solved with a factory reset of the device, according to Apple support.
Since there seemed to be no way we could access the database on the device, I ended up just manually entering the points and their data into the web map. Luckily there were only 26 points on the device, so it wasn't too bad in the grand scheme of things.
However, the underlying issue I was more concerned about was WHY this error happened in the first place. I wasn't the original creator of the map, but I confirmed there were no copies of layers within the web map and everything was configured correctly for offline capability. The data schema and map configuration had not changed since the user downloaded the offline map. I wasn't finding similar situations or solutions documented online, so I was completely stumped as to what the issue was with the data.
After discussing with tech support, we were able to dig up some more information on the issue. The problem that I didn't realize at first, because I wasn't the original publisher of the data or creator of the map, was that the feature service that included the four layers in the web map contained layers that were published from a template geodatabase. One of the feature classes within this template geodatabase was a copy of another feature class with a slightly different schema. This wasn't apparent when looking at the hosted feature service in AGOL.
Even if I hadn't known which geodatabase the service originated from, I could still confirm that two out of the four layers in the service originated from the same data source by downloading the Hosted Feature Layer as a file geodatabase. When I opened the geodatabase, it only contained three instead of four layers. This behavior is expected, according to BUG-000099537, referenced below.
It was recommended by Esri Support to publish layers as separate services if they were intended for offline data collection.
- ENH-000101782 : In the "Publish hosted features layers" documentation for ArcGIS Online, it needs to be mentioned that users need to have each layer pointing to a separate data source if they want to enable the data for offline use.
- ENH-000101849 : If a feature service or hosted feature service is being published to ArcGIS Online or ArcGIS Server and has sync capabilities enabled, ArcGIS Desktop should analyze the map document to see if any layers are pointing to a single data source. If so, the analyze tool should return an error and prevent the user from publishing if they have multiple layers in their map document pointing to the same data source.
- BUG-000099537 : When multiple layers from the same data source are shared as a feature service from ArcMap to ArcGIS Online, exporting the service to a file geodatabase causes only one of the layers from the original data source to be exported.
- This bug may seem unrelated, but this is relevant because it explains that when publishing a service where multiple layers reference the same data layer, only one table is generated in the ArcGIS Online database. ArcGIS Online does not duplicate the tables and therefore it cannot sync data back. ArcGIS Online recognizes multiple layers in one hosted feature layer as one single layer instead of multiple layers. That's why publishing each layer separately is necessary.
So, hopefully that will add to the Geonet collective knowledge and maybe help someone in the future. 
Happy troubleshooting!
