[Koha] Can't call method "branchname" on an undefined value

[lists]

Hi,

A week of two ago we started receiving perl errors when opening items 
from the search results using the OPAC:

Software error:
Can't call method "branchname" on an undefined value at
/usr/share/koha/lib/C4/Biblio.pm line 1708.

In the koha logs we get a bunch of lines like:
> [Mon Mar 27 13:36:21.295319 2017] [cgi:error] [pid 1084] [client
> a.b.c.88:50700] AH01215: [Mon Mar 27 13:36:21 2017] opac-search.pl:
> Use of uninitialized value in hash element at
> /usr/share/koha/lib/C4/XSLT.pm line 294., referer:
> http://opac.company.com/

However using the Admin interface, we CAN open the same records, and 
also check them in/out etc, and the above errors in the koha log files 
do NOT appear..!?

Logged in or out while using the OPAC makes no difference.
We are on debian jessie, koha 16.11.05.000, fully up-to-date.

Tips? Suggestions?

Aditionally, though we don't feel this is related, lately we are getting 
a new checkout confirmation request:

"Please confirm checkout
This patron can't check out this item per library circulation policy.
No circulation rule is defined for this patron and itemtype combination."

We assume that this last message is just new functionality, or a new 
default that we have to look into. Just thought I'd mention it too.

Best regards,
MJ