Sync error: Illegal start of token [<]

[PaulBoehnlein]

Trying to sync a feature service with offline edits in Collector and receiving the error "Illegal start of token [<]".  Service has subtypes, domains and editor tracking.  Hosted in a ArcGIS Online web map.  Can anyone help with this?

[DavideContin]

I have found the same problem with iOS Collector using a map containing some protected services hosted in our own server; from the company network we have no problem syncing the new features but with external connection (wifi or mobile) the synchronization fails with error: "Illegal start of token [<]"

Is this a firewall problem or something else? Anyone know how come does this error happen?

[GaryMorris1]

It might be a problem with the Web Adaptor used when you run Server for ArcGIS on your own host.  If you are still having this problem please open a support ticket with Esri support.  The more support calls Esri gets on this, the more it will help collect enough information to solve the problem. 

The technical reason for the "Illegal start of token" message is that the server (or web adaptor) is responding with an HTML error page (which starts with "<") when the app is expecting JSON reply.  Some users have used wireshark or Charles to view the HTML to find out what error was reported.

[Sravan_KumarUddarraju]

I have had this issue because field type is blob for on one of the layer that are participating in featurelayer. Deleting blob field resolved the "illegal start of token" issue.

[DavideContin]

This is our experience:

With iOS (iPone 4) the sync with external and internal connection works without problems.

With iOS (iPad Air) or Android (Samsung S4 Active) Collector using a map containing some protected services hosted in our own server; from the company network we have no problem syncing the new features but with external connection (wifi or mobile) the synchronization fails with error: "Illegal start of token [<]" (error visible only in iOS, Android doesen't show any error but ends without sync)

We overcame the problem with the port 6080 bypassing the Web Adaptor (adding :6080 to the link of the rest services) but it would be better to understand why this error is generated for iOS (iPad Air) or Android (Samsung S4 Active) but not for iOS (iPone 4); I have opened a technical issue to Esri support.

[SanneMikkelsen]

Im having the same problem in Explorer for ArcGIS ver. 10.2.7. on an Ipad Air.

[AdrianMarsden]

I'm geting this too - but ONLY when the basemaps I am using offline are my own, stored on the iPhone 5.

My basemap is of the same projection as my data, but NOT the same as the esri ones.

Is this likely to be an issue?

[AdrianMarsden]

More research and it appears to work when I have a layer that has not been enabled for attachments, but fails as soon as I enable it for attachments.

[PaulBoehnlein]

I never fully resolved this problem, but did reduce the frequency of the error by increasing the service timeout lengths.

[SamH]

I am currently having this problem, getting the same error message "Sync Error an error occurred while synchronizing edits - illegal start of token [<]" Did anyone ever manage to resolve this issue ?