fts_solr Bad Request / BUG: Unknown internal error

Hi,

yesterday I created a new mailbox; ever since (and only for the new mailbox) I am getting these errors:

2015-06-12T00:00:29.808526+02:00 mail dovecot: indexer-worker(xxx@example.com): Error: fts_solr: Lookup failed: Bad Request 2015-06-12T00:00:29.808560+02:00 mail dovecot: indexer-worker(xxx@example.com): Error: Mailbox INBOX: Status lookup failed: BUG: Unknown internal error

Is this a known issue? Rgds, N.