Unable to serialize JSON: Non-unserializable property set at $ (ParserCache)

[paladox]

Setup

• SMW version: 4.1.2
• MW version: 1.40.1
• PHP version: 7.4
• DB system (MySQL, Blazegraph, etc.) and version: mariadb 10.5

Issue

Detailed description of the issue and a stack trace if applicable:

I seem to get the following:

Unable to serialize JSON: Non-unserializable property set at $

(i've formated the error slightly different because the later part is in a object {'message': ...}

in the logs whilst using semanticmediawiki. Disabling gets rid of this.

Steps to reproduce the observation (recommendation is to use the sandbox):

I've reporduced with https://www.bluepageswiki.org/wiki/Moreauville_Post_Office_(Louisiana) (each time i save, it triggers).

[paladox]

cache keys such as bluepageswiki:pcache:idoptions:44979 and bluepageswiki:pcache:idhash:44979-0!canonical