Error:

Caught exception in MusicBrainz::Server::Controller::Recording->show "Can't call method "id" on an undefined value at lib/MusicBrainz/Server/Entity/Recording.pm line 68."

Stack trace:

line 68 MusicBrainz::Server::Entity::Recording
line 72 MusicBrainz::Server::Entity::Recording
line 89 MusicBrainz::Server::Controller::Recording
line 120 MusicBrainz::Server::Controller::Recording
line 322 MusicBrainz::Server
line 322 MusicBrainz::Server
line 322 MusicBrainz::Server
line 286 MusicBrainz::Server

Request data:

$VAR1 = {
'query_parameters' => {},
'body_parameters' => {}
};

URL: http://musicbrainz.org/recording/da3a9264-13a6-45f5-9c6c-ea24478c7954

and

Error:

Caught exception in MusicBrainz::Server::Controller::Release->show "Can't call method "isa" on an undefined value at lib/MusicBrainz/Server/Controller/Release.pm line 179."

Stack trace:

line 179 MusicBrainz::Server::Controller::Release
line 322 MusicBrainz::Server
line 322 MusicBrainz::Server
line 322 MusicBrainz::Server
line 286 MusicBrainz::Server

Request data:

$VAR1 = {
'query_parameters' => {},
'body_parameters' => {}
};

URL: http://musicbrainz.org/release/127d2cb7-caaa-4e7d-9932-7ff5909d751d

(release containing the recording)

I cancelled the create relationship after that to see, whether the error would go away, but it didn't help.
Clearing all caches, deactivating all userscripts, shift-reload didn't help, either.