After upgrading K2 on server check the following folders - you may see such a problem
administrator/components/com_k2/controllers/
usergroups.php
userGroups.php
usergroup.php
userGroup.php
extrafieldsgroups.php
extraFieldsGroups.php
extrafieldsgroup.php
extraFieldsGroup.php
extrafields.php
extraFields.php
extrafield.php
extraField.php
administrator/components/com_k2/models/
userGroup.xml
usergroup.xml
What is the reason for it? Unlike windows, some Linux servers may consider files in different case to be different and do not overwrite them. So you'll have e.g. controllers for both old and new K2 version.
In K2 version 2.5 corresponding files are lowercase.
If you have such an issue, just delete older files.
Otherwise I suppose that in some cases the possibility of referring extensions to old version files exists and may cause issues.
Original article (in Russian)