Some genomic regions exported in genbank format from Ensembl can't be opened in Ugene. I believe (but haven't extensively checked) that it is due to the presence of features which label makes reference to a remote sequence, eg: join(AC009835.14.1.157979:110651..120793)
The problem existed previously in Artemis but has been corrected recently. I am attaching a file that can't be opened, so you can test by yourself and hopefully correct the bug.
Sincerely,
Sebastien