Joomla 6?

Now I have this error when I call index.php?option=com_sermonspeaker&view=sermons: 0 Class "SermonspeakerHelperSermonspeaker" not found

And this error when I try to add a sermon: 0 Call to undefined method Joomla\Database\Mysqli\MysqliQuery::castAsChar()

Do you have a custom layout? Then you need to adjust that classname in it. It would be called "Sermonspeaker\Component\Sermonspeaker\Site\Helper\SermonspeakerHelper" now.

If it's not a custom layout, which one are you using?

"->castAsChar" was removed in Joomla 6. You have to change it to "->castAs('CHAR', ...)".
But I never used that in SermonSpeaker, so that would be some custom code somewhere. Wait, I found it. It is in the finder plugin which I haven't converted to Joomla 6 yet. So you can disable the sermonspeaker finder plugin  for now and the error should be gone.

If you talk about a custom layout for Sermonspeaker the answer is no.

If it for the Joomla, yes: I use Gantry 5.

Thomas Hunziker wrote:

"->castAsChar" was removed in Joomla 6. You have to change it to "->castAs('CHAR', ...)".
But I never used that in SermonSpeaker, so that would be some custom code somewhere.

I'll have to look it up because I don't know what it is or where it might be. 

Which Layout are you using for the sermons list? Table, list, blog, tiles or columns.
What happens if you choose a different layout in the SermonSpeaker options?

Maybe you also once accidentaly created an override in the Joomla Override manager. Then you can just delete that one there.