I try to provide some kind of single-sign-on experience for my Nextcloud and Joomla users. Nextcloud is able to read out all users from the Joomla users SQL table, but I am not able to login with a joomla user to Nextcloud.
I made some tests with a mockup joomla user database and it turns out that “plain text” passwords seem to work. So to me it looks like that the used password hash method is not compatible with Joomla. I tried
Joomla MD5 (as this was an obvious choice to try) and
Blowfish (because I read it somewhere)
I personally don’t use Joomla but I can imagine that an external program, like Nextcloud, could have problems when directly accessing password data. Therefore I would recommend to reconfigure Joomla so that it acts as an OAuth2 server and then let Nextcloud connect to it by using Oauth2 authentication. This is e.g. provided by the Social Login app.
BTW, the other way round would also possible, because Nextcloud can act as an Oauth2 server out of the box. You would only have to find an Oauth2 client plugin for Joomla
Unfortunately you only provided a shortened log message which doesn’t provide further information which might help to narrow down the problem. Perhaps the following issue tickets gives you an idea how a detailed problem report should look like and how ta problem can be troubleshooted.