Next/Previous links fail in forum after upgrading to 4.5.6

As stated in the forum, after yesterday's upgrade to 4.5.6, the "Previous topic" and "Next topic" links in forum posts act a bit strangely.

They will work to browse a few posts, and at some point (maybe the 5th or so), you'll get an error message like:

Fatal error: Cannot create references to/from string offsets nor overloaded objects in /srv/www/lliurex/http/drupal-4.5.6/modules/filter.module on line 1023

If you then try to reload that page, you'll eventually get the node loaded after a few tries. When you do, it's common that you next try to go forward to the next post will fail again similarly, with the same error message.