I'm not sure if this is intermittent or solid ... I'll go back and check. However, here is the database error message for you to investigate ...

Mysql error at /showpic/output/page/noclick//image/media%2Fforum_uploads%2F5_10%2Flib022_1273969680_635.jpg/return/%2Fwall

Told to get one row. Query was:

"select id, filename from forum_files where id > and filename != '5_10/lib022_1273969680_635.jpg' order by id asc".

Mysql said You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'and filename != '5_10/lib022_1273969680_635.jpg' order by id asc' at line 1referring url was https://umd.net/wall
Mysql error at /showpic/output/page/noclick//image/media%2Fforum_uploads%2F5_10%2Flib022_1273969680_635.jpg/return/%2Fwall

Told to get one row. Query was:

"select id, filename from forum_files where id < and filename != '5_10/lib022_1273969680_635.jpg' order by id desc".

Mysql said You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'and filename != '5_10/lib022_1273969680_635.jpg' order by id desc' at line 1referring url was https://umd.net/wall

It is reproducible. Here is the URL that got me there...

apparently I can't past a full URL ??? ... here it is without the protocol and host part.
/showpic/output/page/noclick//image/media%2Fforum_uploads%2F5_10%2Flib022_1273969680_635.jpg/return/%2Fwall