SkinManager error - what does that mean?

During the last days I found several error messages in server.log regarding "SkinManager":

[19-11-10 14:13:50.8753] Slim::Utils::Misc::msg (1256) Warning: [14:13:50.8745] Use of uninitialized value $name in hash element at /usr/local/share/perl5/vendor_perl/Template/Context.pm line 100.
[19-11-10 14:13:50.8763] Slim::Utils::Misc::msg (1256) Warning: [14:13:50.8756] Use of uninitialized value $shortname in substitution (s///) at /usr/local/share/perl5/vendor_perl/Template/Context.pm line 116.
[19-11-10 14:13:50.8774] Slim::Utils::Misc::msg (1256) Warning: [14:13:50.8767] Use of uninitialized value $name in concatenation (.) or string at /usr/local/share/perl5/vendor_perl/Template/Context.pm line 168.
[19-11-10 14:13:50.8784] Slim::Web::Template::SkinManager::_fillTemplate (346) Error: file error - : not found

Does anyone know what this means and how to solve this problem?