[TYPO3-project-seminars] Fatal (PHP) errors in BE

[Sacha Storz]

Hi there. :)

A have successfully installed seminars with TYPO3 4.3. in the past (for 
the record ;).

Now I installed on a fresh and working 4.3 installation, doing 
everything in the exact same way (I think) and it's a disaster. I keep 
getting fatal PHP errors in the BE. For example:

When I try to add a date to the according sysfolder in the LIST module I 
get: Fatal error: Call to a member function isOk() on a non-object in 
/srv/www/htdocs/www.MYDOMAIN.de/path/typo3conf/ext/seminars/class.tx_seminars_tcemain.php 
on line 163Fatal

When I click on the sysfolder containing topics (which I was able to 
generate via LIST module) in the SEMINARS module I get: Fatal error: 
Call to a member function checkConfiguration() on a non-object in 
/srv/www/htdocs/www.MYDOMAIN.de/path/typo3conf/ext/seminars/mod1/index.php 
on line 275

And many more errors. :( I checked everything I know of and imho I 
followed the manual to the dot, I finally uninstalled, reinstalled. Same 
errors. What did I miss?!

I use:
* TYPO3 Ver. 4.3.0
* PHP 5.2.0-8+etch16
* seminars  0.8.3
* ameos_formidable  1.1.0
* oelib  0.6.3
* onetimeaccount  0.6.1

plugin.tx_seminars_pi1 {
    attendancesPID = 31
    registerPID = 22
    thankYouAfterRegistrationPID = 23
    pageToShowAfterUnregistrationPID = 33
    listPID = 7
    detailPID = 24
    loginPID = 25
    templateFile = fileadmin/somepath/seminars_pi1.tmpl
    enableRegistration = 1
}

ANY help/hint/suggestion would be highly appreciated.
Thanks a lot in advance
sacha