Hi,
there seems to be an issue with the mastodon instance requiring AUTHORIZED_FETCH
.
Lemmy 0.19.3 does currently not have a working implementation of authorized fetch, this should get fixed in 0.19.4 when that is released.
For now, the only workaround is to disable secure mode on mastodon.
edit: see also https://github.com/LemmyNet/lemmy/issues/4451