Hi, I'm getting the same error with webgisdr. The item does exist but fails to find it. In my case the content directory is on a shared location. How many '\\\\' should it have?
Failed to back up the Portal for ArcGIS:
Url: https://my.portal.url/portal.
{"error":{"code":500,"details":null,"message":"Failed to export site. Export of the Portal repository failed. java.lang.Exception: Source file does not exist or is not a file. File path = \\\\my.share.directory\\fileshare01\\arcgisportal\\content\\items\\1df24126fb13421597ae69739bda14f2\\esriinfo\\thumbnail\\ago_downloaded.png"}}
This is how the content directory is specified on the portaladmin:
{"provider":"FileSystem","connectionString":"\\\\my.share.directory\\fileshare01\\arcgisportal\\content","type":"fileStore"}