I've now upgraded to version 0.9 and re-imported all my contacts from my GMail account. Previously, the URLs were okay, but now they have additional backslashes before some characters. All URLs are now "http\://..." which obviously doesn't work. They are already like this in the .vcf file, so Google exports them like that. I don't know the inner details of the vCard format, so is it correct to write backslashes there and should RoundCube fix it, or does Google export buggy files and should I report the error to them?
vCard specification doesn't require colon escaping with a backslash. However, if you open a ticket in our bugtracker we'd think about adding a workaround. Currently backslash need to be escaped, so we could detect any "single backslash" and ignore it.
Created ticket: http://trac.roundcube.net/ticket/1489085