Thanks Gerster for your error report.
You don't say what version of StageShow you are using, but assuming it is 2.1.6, then the code at stageshowlib_salesplugin.php line 584 is as follows:
$rslt->paypalParams = array();
This includes an implicit creation of a StdClass object (as $rslt) and it appears that this is generating a PHP warning. As far as I can tell this Warning is only generated if the PHP E_STRICT reporting level is set, which is not usually the case for production servers. I actually thought that WordPress turned off E_STRICT anyway, so why the warning is generated is a bit of a mystery.
The checkout should redirect to the PayPal page, and this cannot happen if any output has been generated, in your case the PHP Warning has already been output so the redirect fails.
As for a fix .... you could try adding code before the line generating the error to explicitly create the object, as follows:
$rslt = new stdClass();
Alternatively, if E_STRICT is turned on, turn it off.
I've added this change to my core code, so this will be included in future releases.