Calendar synchronisation fails via mobile internet & wifi : "No internet connection"

[fnx2k]

I have been using DAVx5 for a few years now. I used it on the same mobile device with no major update in the last years.
Calendar sync was working smoothly as expected.

Sometime about 2023, after a version update (sorry I did not note the version), synchronizing over mobile internet has been persistently failing. I thought it was a transitory bug.
However, the issue remained with all subsequent versions.

I could sync only when the phone was connected over Wifi.

With version 4.4.3.2-ose, connection via Wifi AND Mobile internet fails.

I downgraded my installed version to 4.4.2-ose to keep at least one way of syncing.

Problem scope

Calendar sync.

App version

• 4.4.3.2-ose --> sync fails for Wifi and Mobile Internet
• 4.4.2-ose & 4.4.1.1-ose --> sync fails for Mobile internet

Android version and device/firmware type

OS Android 10
Device: Honor 10, with EMUI 10.0.0

Steps to reproduce

Set up a CalDav
Disconnect Wifi
On the page for sync error message is shown: No internet connection

Attempted troubleshooting

I have read thoughtfully faq/synchronization-is-not-run-as-expected

I have checked the following parameters for app permissions
![Screenshot_at bitfire davdroid]

[Capture from network_permissions_com huawei systemmanager]

In this sequence, wifi can be swith on / off and DavX5 hide or show the error message.

Here are some debug logs:

• for version: 4.4.2-ose (sync in Wifi only)
  davx5_debug_ v4.4.2-ose_1.zip

• for version: 4.4.3.2-ose (not syncing)
  davx5_debug_log3_V4.4.3.2-ose.zip

Version id:
Screenshot_version_at bitfire davdroid

[rfc2822]

• for version: 4.4.3.2-ose (not syncing)
  davx5_debug_log3_V4.4.3.2-ose.zip

The debug info shows some problems:

App exempted from power saving: no (bad)

The app should be exempted from power saving (see FAQ). Otherwise synchronization is affected.

System-wide synchronization: manually

The "Sync data" setting (usually somewhere in Android Settings / Accounts) is disabled. This doesn't really matter, but it would disable syncs on local changes. The regular sync (with the defined interval) is however not affected.

☒ rmnet0

• [ Transports: CELLULAR Capabilities: INTERNET NOT_RESTRICTED TRUSTED NOT_VPN NOT_ROAMING FOREGROUND NOT_CONGESTED NOT_SUSPENDED LinkUpBandwidth>=51200Kbps LinkDnBandwidth>=102400Kbps Specifier: <****>]

There is no VALIDATED flag (although INTERNET is present). This means that the Android system couldn't use this connection to validate Internet access by doing some HTTP request without data. Then Android won't advertise this connection as usable and will not run jobs that require Internet, like DAVx5 synchronization. Is there some active firewall or other things that might block Internet access for some processes?

There's also no WiFi connection in the list, so no WiFi was present at the time of creating the debug info. So I can't say anything about the WiFi.

for version: 4.4.2-ose (sync in Wifi only)
davx5_debug_ v4.4.2-ose_1.zip

There's a VALIDATED INTERNET connection for the WiFi:

☒ wlan0
   - [ Transports: WIFI Capabilities: NOT_METERED INTERNET NOT_RESTRICTED TRUSTED NOT_VPN VALIDATED NOT_ROAMING FOREGROUND NOT_CONGESTED NOT_SUSPENDED LinkUpBandwidth>=1048576Kbps LinkDnBandwidth>=1048576Kbps SignalStrength: -85]

So it can be used. There's no information about the cellular connection because you're connected to WiFi. But the problem is probably the same as above.

Usually this is caused by some firewall that breaks Android Internet detection.