OIDplus error

Using $this when not in object context

Technical information about the problem:

Error
string(85) "/var/www/vhosts/registry.frdl.de/httpdocs/includes/classes/OIDplusAuthUtils.class.php"
int(251)
at file /var/www/vhosts/registry.frdl.de/httpdocs/includes/classes/OIDplusAuthUtils.class.php (line 251)
Stacktrace:
#0 /var/www/vhosts/registry.frdl.de/httpdocs/plugins/objectTypes/host/OIDplusHost.class.php(137): OIDplusAuthUtils::isAdminLoggedIn()
#1 /var/www/vhosts/registry.frdl.de/httpdocs/plugins/publicPages/000_objects/OIDplusPagePublicObjects.class.php(491): OIDplusHost->getContentPage()
#2 /var/www/vhosts/registry.frdl.de/httpdocs/includes/classes/OIDplusGui.class.php(34): OIDplusPagePublicObjects->gui()
#3 /var/www/vhosts/registry.frdl.de/httpdocs/index.php(31): OIDplusGui::generateContentPage()
#4 {main}