Interesting enough is, that I have the older shop version with an older Joomla version running in a sub-folder and that works fine.
Therefore I thought anybody in this forum could help.... ?
Yes I have contacted the hosting provider. The error is:
<p>The server encountered an internal error or
misconfiguration and was unable to complete
<p>Please contact the server administrator,
[no address given] and inform them of the time the error occurred,
and anything you might have done that may have
caused the error.</p>
<p>More information about this error may be available
in the server error log.</p>