Not possible to convert lead to contact incl. creating case and link to campaign !

Not possible to convert lead to contact incl. creating case and link to campaign !

Hello,

Actually, it seems not longer possible to convert a lead to contact incl. creating case and link to campaign !
Once I submit the conversion, the page is empty and the browser hangs for hours.
So I checked without linking a campaign, but creating a case. It took about a minute to display the contact resulting from the conversion. What looks like the half eternity. But the account and case have been created. Linking to campaign manually worked too...

Regards
Christian