Mac – iCloud requests fail with timeout after a few minutes of connectivity

When connecting to a wifi network and waiting for a short period of time (ranging from 20 seconds to 5 minutes), certain iCloud related requests will result in a timeout.

I found the error first in BusyCal, a third party calendar app, when trying to sync iCloud calendars, but it also occurs in the native calendar app. Both apps try syncing for about 20 seconds and give an error sign afterwards. The error even manages to beachball system preference panes for iCloud and internet accounts (sometimes).

I inspected console and found that there are requests failing with a timeout (in German: "Zeitüberschreitung"). Here is a log item for the process com.apple.iCloudHelper:

AOSKit ERROR: Storage usage request failed, appleID=MY_APPLE_ID, url=https://p29-quota.icloud.com:443/quotaservice/external/osx/1060093596/storageUsageInfo, requestHeaders=
{
    "Accept-Language" = "de-de";
    Authorization = "...";
    "X-Mme-Client-Info" = "<MacBookPro12,1> <Mac OS X;10.12.6;16G1114> <com.apple.AOSKit/252 (com.apple.CloudPhotosConfiguration/2.0)>";
    "X-Mme-Country" = DE;
    "X-Mme-Device-Id" = "191F7B8A-ED81-5E19-85BD-5690FA246D47";
    "X-Mme-Timezone" = MEZ;
},
error=Error Domain=AOSErrorDomain Code=1000 "(null)" UserInfo={UnderlyingError=Error Domain=kCFErrorDomainCFNetwork Code=-1001 "Zeitüberschreitung bei der Anforderung." UserInfo={NSErrorFailingURLStringKey=https://p29-quota.icloud.com:443/quotaservice/external/osx/1060093596/storageUsageInfo, NSErrorFailingURLKey=https://p29-quota.icloud.com:443/quotaservice/external/osx/1060093596/storageUsageInfo, _kCFStreamErrorCodeKey=-2102, _kCFStreamErrorDomainKey=4, NSLocalizedDescription=Zeitüberschreitung bei der Anforde

Here is another log item for process CalendarAgent:

[com.apple.calendar.store.log.caldav.mismatch] [Sync report for calendar home iCloud, failed with error Error Domain=NSURLErrorDomain Code=-1001 "Zeitüberschreitung bei der Anforderung." UserInfo={NSUnderlyingError=0x7fa1138a5c40 {Error Domain=kCFErrorDomainCFNetwork Code=-1001 "Zeitüberschreitung bei der Anforderung." UserInfo={NSErrorFailingURLStringKey=https://MY_APPLE_ID@p29-caldav.icloud.com/1060093596/calendars/, NSErrorFailingURLKey=https://MY_APPLE_ID@p29-caldav.icloud.com/1060093596/calendars/, _kCFStreamErrorCodeKey=-2102, _kCFStreamErrorDomainKey=4, NSLocalizedDescription=Zeitüberschreitung bei der Anforderung.}}, NSErrorFailingURLStringKey=https://MY_APPLE_ID@p29-caldav.icloud.com/1060093596/calendars/, NSErrorFailingURLKey=https://MY_APPLE_ID@p29-caldav.icloud.com/1060093596/calendars/, _kCFStreamErrorDomainKey=4, _kCFStreamErrorCodeKey=-2102, NSLocalizedDescription=Zeitüberschreitung bei der Anforderung.}]

(If there's anything too confidential in there, please let me know.)

The error occurs on 2 different wifi networks. Once I disable and reenable wifi, everything works for a short period of time until the error occurs again. Error is wifi only and doesn't occur when tethered to an iPhone's 3G connection.

Things I tried so far without success:

• deleting /Users/MY_USER_NAME/Library/Preferences/MobileMeAccounts.plist,
• rebooting in safe mode and
• wondering what I might have done to deserve this ?

I'm on a MacBook Pro with Sierra (10.12.6). Happy to provide more info if needed.

Any ideas? I'm pretty desperate over here.