Thanks again for your help !
After investigation, the issue came after an upgrade of the web server (nginx) from the release 1.13.5 to the 1.13.6 one.
But I suspect again a bug in Android 7.0 because on Thunderbird and on Android 6.0 (with the last release of DAVdroid), there were no issue after the nginx upgrade.
I think I will downgrade my smartphone (Samsung A5 2016) to Android 6 because Samsung didn’t generate an update to Android 7.1 for this device.
Have a good week-end.