For posterity, in case that topic has multiple custom fields you should use:
tcf = TopicCustomField.where(plugin_name: 'discourse-events', topic_id: problematic_topic_id)
c = tcf.first
c.value = "[295,50]"
c.save
@hellekin I would migrate to 🗓 Discourse Event.
In fact, perhaps you could work with @fzngagan on a migration script from Events to Discourse Events? It’d be good to have a real case to work with there. Faiz, we should prioritise this, I’d like to formally deprecate Events.