Thanks a lot - the sync behavior is much better now.
After a couple of sync processes only a remainder of 7 contacts fail to sync. - the other contacts remain on the phone.
Could it have something to do with the contacts?
The link addresses from the log file work fine when I do open them in firefox.
12-19 21:12:29.176 11849-30099/at.bitfire.davdroid:sync I/davdroid: [syncadapter.ContactsSyncManager] Downloading 7 contacts (10 at once)
12-19 21:12:29.177 11849-30099/at.bitfire.davdroid:sync I/davdroid: [syncadapter.ContactsSyncManager] Downloading <address> 17b7-40db-b485-44b63a405cff.vcf, <address>d02f-4bf8-b1f0-c826a12e1446.vcf, <address>7daf-4097-9b95-125fb9625b4d.vcf, <address>256A677E-5F6F88CE.vcf, <address>1755-4e62-ae21-52406896a863.vcf, <address>4f49-b1db-fb1b9d581cdf.vcf, <address>0364-4e68-bfc4-374089d492a3.vcf
12-19 21:12:44.550 11849-11866/at.bitfire.davdroid:sync W/art: Suspending all threads took: 18.278ms
12-19 21:12:59.060 11849-11866/at.bitfire.davdroid:sync W/art: Suspending all threads took: 23.914ms
12-19 21:13:47.559 11849-11866/at.bitfire.davdroid:sync W/art: Suspending all threads took: 5.277ms
12-19 21:13:52.061 11849-11866/at.bitfire.davdroid:sync W/art: Suspending all threads took: 7.325ms
12-19 21:14:29.185 11849-30099/at.bitfire.davdroid:sync W/davdroid: [syncadapter.SyncManager] I/O exception during sync, trying again later
EXCEPTION java.net.SocketTimeoutException: timeout
at okio.Okio$4.newTimeoutException(Okio.java:227)
at okio.AsyncTimeout.exit(AsyncTimeout.java:274)
at okio.AsyncTimeout$2.read(AsyncTimeout.java:242)
at okio.RealBufferedSource.indexOf(RealBufferedSource.java:325)
at okio.RealBufferedSource.indexOf(RealBufferedSource.java:314)
at okio.RealBufferedSource.readUtf8LineStrict(RealBufferedSource.java:210)
at okhttp3.internal.http1.Http1Codec.readResponse(Http1Codec.java:191)
at okhttp3.internal.http1.Http1Codec.readResponseHeaders(Http1Codec.java:132)
at okhttp3.internal.http.CallServerInterceptor.intercept(CallServerInterceptor.java:54)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:92)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:67)
at at.bitfire.dav4android.BasicDigestAuthHandler.intercept(BasicDigestAuthHandler.java:290)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:92)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:67)
at at.bitfire.davdroid.HttpClient$UserAgentInterceptor.intercept(HttpClient.java:164)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:92)
at okhttp3.internal.connection.ConnectInterceptor.intercept(ConnectInterceptor.java:45)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:92)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:67)
at okhttp3.internal.cache.CacheInterceptor.intercept(CacheInterceptor.java:93)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:92)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:67)
at okhttp3.internal.http.BridgeInterceptor.intercept(BridgeInterceptor.java:93)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:92)
at okhttp3.internal.http.RetryAndFollowUpInterceptor.intercept(RetryAndFollowUpInterceptor.java:120)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:92)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:67)
at okhttp3.RealCall.getResponseWithInterceptorChain(RealCall.java:179)
at okhttp3.RealCall.execute(RealCall.java:63)
at at.bitfire.dav4android.DavAddressBook.multiget(DavAddressBook.java:111)
at at.bitfire.davdroid.syncadapter.ContactsSyncManager.downloadRemote(ContactsSyncManager.java:327)
at at.bitfire.davdroid.syncadapter.SyncManager.performSync(SyncManager.java:170)
at at.bitfire.davdroid.syncadapter.ContactsSyncAdapterService$ContactsSyncAdapter.onPerformSync(ContactsSyncAdapterService.java:66)
at android.content.AbstractThreadedSyncAdapter$SyncThread.run(AbstractThreadedSyncAdapter.java:259)
12-19 21:14:29.186 11849-30099/at.bitfire.davdroid:sync I/davdroid: [syncadapter.ContactsSyncAdapterService$ContactsSyncAdapter] Address book sync complete
12-19 21:16:29.431 11849-11866/at.bitfire.davdroid:sync W/art: Suspending all threads took: 8.461ms