Interestingly enough this is happening only first time (item get checked in and then this "Internal Server Error" blank page appears). If I get back to /cgi-bin/koha/circ/branchtransfers.pl and put the same barcode second time, the transfer is processed without any further error. Possible relation to the Bug 3536 - Checked In item requiring transfer does not consistently trigger transfer prompt (https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=3536)
It could help to know the error you have in the Koha log file.
Which exact version of Koha?
Could it be the item was missing a holdingbranch or something like that? But Jonathan is right, without more information (steps we can replicate) or at least the error from the logs, I am not sure how we can help here.
The reporter hasn't provided any additional information - closing for now.