Magento: Blank page on admin url - magento

Question

I upgraded from 1.9 to 1.9.1 via MagentoConnectManager. Everthing seems to work well and the site looks good. But the admin-page ist just a blank white page. I checked the .htaccess and the core_config_data table changed from http to https but nothing is working.
The exeption log shows (Links anonym):
xxxurlxxx ...app/Mage.php:595
Stack trace:
#0 xxxurlxxx ...app/code/core/Mage/Api/Model/Server.php(127): Mage::throwException('Invalid webserv...')
#1 xxxurlxxx ...api.php(78): Mage_Api_Model_Server->initialize(NULL)
#2 {main}
2015-02-28T00:31:47+00:00 ERR (3):
exception 'Mage_Core_Exception' with message 'Invalid webservice adapter specified.' in xxxurlxxx ...app/Mage.php:595
Stack trace:
#0 xxxurlxxx ...app/code/core/Mage/Api/Model/Server.php(127): Mage::throwException('Invalid webserv...')
#1 xxxurlxxx ...api.php(78): Mage_Api_Model_Server->initialize(NULL)
#2 {main}
2015-03-06T10:03:41+00:00 ERR (3):
exception 'Mage_Core_Exception' with message 'Invalid webservice adapter specified.' in xxxurlxxx ...app/Mage.php:595
Stack trace:
#0 xxxurlxxx ...app/code/core/Mage/Api/Model/Server.php(127): Mage::throwException('Invalid webserv...')
#1 xxxurlxxx ...api.php(78): Mage_Api_Model_Server->initialize(NULL)
#2 {main}
Can anybody please help and give me a hind what to do?
The page is h**p://....vinocada.de bye the way.
Best regards,
Markus