Trying Upgrading again gives following result and keeps version at 5.5.0a:
Are you sure you want to upgrade the 1 selected scripts to their latest versions?
SugarCRM - Upgrading from 5.5.0a to 5.5.1
Upgrading SugarCRM to version 5.5.1 ..
Configuring Apache PHP settings ..
.. already done.
Found http://www.sugarforge.org:80/frs/download.php/6592/SugarCE-5.5.1.zip in cache ..
Minor version upgrades must be done within SugarCRM
.. failed! See the error message above for the reason why.
Then trying to access the sugarcrm gives:
Sugar CRM 5.5.0a Files May Only Be Used With A Sugar CRM 5.5.0a Database.
(not a critical for us, as we don't use it anymore temporarly, but still needs resolution).