OMP error when browsing series or categories

Hi all! I’ve been using OMP for 2 months. It works very well, but I’m having a specific problem. If I try to browse a series or category, the system shows a blank page with the message:

"DB Error: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list LINE 1: …f.seq is null then 1 else 0 end, “psf”.“seq” ASC, “st”."sett… ^"

I am running OMP, PHP 5.5.9 and PostgreSQL 9.3.24. I have not upgraded from previous versions. The “error.log” file shows no message when the error occurs.

Has anybody seen this problem and knows how to solve it?