Hi Anne-Sophie!
This message is especially interesting:
The /aows here is most likely the problem. Is your Mattermost installation configured to use a subfolder on your server, like https://yourserver.com/aows/? What is the URL you can reach the Mattermost login at? Is that https://yourserver.com or https://yourserver.com/aows?
The URL it’s trying to access here is necessary for the client to configure itself, so you can manually try to validate it using the following command (I ran this command against Mattermost’s community server, just put your real URL there, ideally exactly the one out of the error message above):
$ curl -s https://community.mattermost.com/api/v4/config/client?format=old | jq .
{
"AboutLink": "https://about.mattermost.com/default-about/",
"AndroidAppDownloadLink": "https://about.mattermost.com/mattermost-android-app/",
"AndroidLatestVersion": "",
"AndroidMinVersion": "",
"AppDownloadLink": "https://about.mattermost.com/downloads/",
"AsymmetricSigningPublicKey": "MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEAxVH0LufA1VFQv3Bp5mgckEsZOvEN/VNW3d3sb6Vf3ALO7NGhehajX58st2Ic+xvmBEDnSnXkq74ZVbyAjUQiQ==",
"BuildDate": "Fri Jun 23 19:15:13 UTC 2023",
"BuildEnterpriseReady": "true",
"BuildHash": "936e388aacf0c53ce77342a21de48fbad270d3cc",
"BuildHashEnterprise": "373a9ec0d566fa4ade017acf4f7f7ca440adb1d1",
"BuildNumber": "master",
"CWSURL": "",
"CustomBrandText": "",
[...]
If you can get a JSON struct back like the one in my example, then you should be good to go. Could you compare it to the one from the community server or post the anonymized version here so I can have a look at possible reasons for the parsing problems?