I didn’t fully understand that sentence. You have 2 remotes, mailboxorg and your own selfhosted (you say Apache, but the murena nextcloud selfhost package would run nginx).
- murenaio itself cannot work atm with Seedvault if davx5 (and SV builtin webdav client) are used, see Backup recommondations - #5 by tcecyk
- mailboxorg dav - unsure what stack they run, forums are at https://userforum.mailbox.org/
if you check adb logcat while the seedvault error is shown you might get more output. Atm I cannot recommend using webdav as Seedvault backup target, only sdcard / local, and sync from there with another App outside Seedvault