On Ubuntu 18.04 freshly installed, I can sync my OwnCloud calendar using gnome-online-accounts. I get the following logs on the OwnCloud server :
```
10.231.136.146 - erdnaxe [13/Apr/2020:13:15:25 +0200] "PROPFIND /remote.php/caldav/calendars/erdnaxe/a-dmnage/ HTTP/1.0" 207 402 "-" "Evolution/3.28.5"
10.231.136.146 - erdnaxe [13/Apr/2020:13:15:25 +0200] "PROPFIND /remote.php/caldav/calendars/erdnaxe/eea-m1-2019-2020/ HTTP/1.0" 207 411 "-" "Evolution/3.28.5"
10.231.136.146 - erdnaxe [13/Apr/2020:13:15:25 +0200] "PROPFIND /remote.php/caldav/calendars/erdnaxe/eea-m1-2019-2020-2nd-semestre/ HTTP/1.0" 207 424 "-" "Evolution/3.28.5"
10.231.136.146 - erdnaxe [13/Apr/2020:13:15:25 +0200] "PROPFIND /remote.php/caldav/calendars/erdnaxe/personal/ HTTP/1.0" 207 403 "-" "Evolution/3.28.5"
10.231.136.146 - erdnaxe [13/Apr/2020:13:21:18 +0200] "PROPFIND /remote.php/caldav/calendars/erdnaxe/personal/ HTTP/1.0" 207 403 "-" "Evolution/3.28.5"
10.231.136.146 - erdnaxe [13/Apr/2020:13:21:18 +0200] "PROPFIND /remote.php/caldav/calendars/erdnaxe/eea-m1-2019-2020-2nd-semestre/ HTTP/1.0" 207 424 "-" "Evolution/3.28.5"
10.231.136.146 - erdnaxe [13/Apr/2020:13:21:19 +0200] "PROPFIND /remote.php/caldav/calendars/erdnaxe/eea-m1-2019-2020/ HTTP/1.0" 207 411 "-" "Evolution/3.28.5"
10.231.136.146 - erdnaxe [13/Apr/2020:13:21:19 +0200] "PROPFIND /remote.php/caldav/calendars/erdnaxe/a-dmnage/ HTTP/1.0" 207 402 "-" "Evolution/3.28.5"
```
On Ubuntu 20.04 beta, also freshly installed, I am unable to sync my OwnCloud calendar using gnome-online-accounts. As I am seeing nothing server-side, I ran a Wireshark capture and got the following (after turning on calendar sync in gnome preferences) :
```
No. Time Source Destination Protocol Length Info
1182 24.482539317 2a0c:700:0:21:cca9:b573:e4da:fa08 2a0c:700:0:24:ba:ccff:feda:aa00 HTTP 355 GET /remote.php/dav/calendars/erdnaxe/eea-m1-2019-2020-2nd-semestre/ HTTP/1.1
1184 24.484320850 2a0c:700:0:24:ba:ccff:feda:aa00 2a0c:700:0:21:cca9:b573:e4da:fa08 HTTP 509 HTTP/1.1 400 Bad Request (text/html)
1193 24.497309577 2a0c:700:0:21:cca9:b573:e4da:fa08 2a0c:700:0:24:ba:ccff:feda:aa00 HTTP 334 GET /remote.php/dav/calendars/erdnaxe/a-dmnage/ HTTP/1.1
1195 24.498695807 2a0c:700:0:24:ba:ccff:feda:aa00 2a0c:700:0:21:cca9:b573:e4da:fa08 HTTP 509 HTTP/1.1 400 Bad Request (text/html)
1205 24.509317111 2a0c:700:0:21:cca9:b573:e4da:fa08 2a0c:700:0:24:ba:ccff:feda:aa00 HTTP 334 GET /remote.php/dav/calendars/erdnaxe/personal/ HTTP/1.1
1207 24.510384332 2a0c:700:0:24:ba:ccff:feda:aa00 2a0c:700:0:21:cca9:b573:e4da:fa08 HTTP 509 HTTP/1.1 400 Bad Request (text/html)
1216 24.520918918 2a0c:700:0:21:cca9:b573:e4da:fa08 2a0c:700:0:24:ba:ccff:feda:aa00 HTTP 342 GET /remote.php/dav/calendars/erdnaxe/eea-m1-2019-2020/ HTTP/1.1
1218 24.521809118 2a0c:700:0:24:ba:ccff:feda:aa00 2a0c:700:0:21:cca9:b573:e4da:fa08 HTTP 509 HTTP/1.1 400 Bad Request (text/html)
1366 27.525341056 2a0c:700:0:21:cca9:b573:e4da:fa08 2a0c:700:0:24:ba:ccff:feda:aa00 HTTP/XML 313 PROPFIND /remote.php/dav HTTP/1.1
1369 27.526366204 2a0c:700:0:24:ba:ccff:feda:aa00 2a0c:700:0:21:cca9:b573:e4da:fa08 HTTP 458 HTTP/1.1 302 Moved Temporarily (text/html)
```
As you can see OwnCloud respond with a Bed Request when using a "GET" HTTP method rather than a "PROPFIND" HTTP method.
Manually inputting full Caldav URI in Gnome Calendar works (and send PROPFIND).
Browsing `davs://owncloud.crans.org/remote.php/dav/calendars/erdnaxe/personnal/` with Nautilus works.
ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: gnome-calendar 3.36.0-1
ProcVersionSignature: Ubuntu 5.4.0-21.25-generic 5.4.27
Uname: Linux 5.4.0-21-generic x86_64
ApportVersion: 2.20.11-0ubuntu26
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Mon Apr 13 17:41:46 2020
InstallationDate: Installed on 2020-04-13 (0 days ago)
InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Beta amd64 (20200402)
ProcEnviron:
PATH=(custom, no user)
XDG_RUNTIME_DIR=<set>
LANG=fr_FR.UTF-8
SHELL=/bin/zsh
SourcePackage: gnome-calendar
UpgradeStatus: No upgrade log present (probably fresh install)
Thank you for your bug report, that looks similar to bug #1871019
There is an upstream fix on https:/ /gitlab. gnome.org/ GNOME/evolution -data-server/ -/issues/ 204 which would be worth testing, I will try to include that in a an update or ppa for testing