Just installed 1.9.10-ose from the fdroid repository: Ressource detection took about 4-6 Min, but worked straight away both with CalDAV and with CARDDav. (I then upgraded to 2.0.3-ose maually via fdroid).
So in conclusion something in the present 2.0.3.-ose ressource detection does not work with both Synology’s Carddav as well as CalDavServer (please note I use an older Synology DSM version using the WebDAV server package for CalDAV).
Hope this and my debug logs above help to find a solution for other Synology users. Until then, the workaround with 1.9.10-ose works.