Peter,
I took a look at your map and reproduced the download error. For your reference and others in this thread, logging can be enabled in the new Collector:
Open the settings > tap your login name at the top 3 times > "Developer" options should appear at the bottom of the settings > Open the Developer options and turn on logging > reproduce the download failure > back in the Developer options, click Logs > you will see more detailed errors that you can review or share out via email, etc.
In Peter's map, and error is being thrown on the "harvest_lines" layer, "The item to be created already exists in the database". This error sounds a lot like a known issue regarding duplicate domain names. I'm guessing you have similar domains applied across the points and lines layers. A simple test would be to remove one of these layers and confirm you can download the map area. If that works, the next step is to investigate the domains. Were they created in ArcGIS online (AGOL) using the list functionality or in Desktop? If you created them in AGOL it applies the field name to the domain name, ie if your field name is "status" the name would be status_domain. If you have another layer with the same field name and you create a domain list for that as well the domain names will be duplicated. When downloaded they are packaged into the same database and the duplicate domains will cause an error. A fix was already applied to AGOL for this but for existing data the lists will need to be modified so they can be renamed with the new scheme, which avoids the potential for duplicates. You can simply open the lists, apply a space to one of the values, remove the space, then save. The new domain name should be applied.
For others, see if you can capture a more specific error using logging and we can look into it. Thanks
Colin
Regards,
Colin