Thank you all.
It does seem to have been corrected in the current trunk:
https://svn.roundcube.net/trunk/roundcubemail/program/localization/da_DK/So if anyone else experiences problems with a blank page after login and the following errors in the logfile, downloading the new labels.inc and messages.inc might fix the problem. Like rosali wrote it can also be fixed by deleting any characters outside the tags.
[noparse]
PHP Warning: Cannot modify header information - headers already sent by (output started at /.../program/localization/da_DK/labels.inc:268) in Unknown on line 0
PHP Warning: Cannot modify header information - headers already sent by (output started at /.../program/localization/da_DK/labels.inc:268) in /.../program/include/rcube_template.php on line 264
PHP Warning: Cannot modify header information - headers already sent by (output started at /.../program/localization/da_DK/labels.inc:268) in /.../program/include/rcmail.php on line 954
[/noparse]
Thanks,
Mads