Access Key won’t save / update

We are running the AntiSpam by CleanTalk plugin on a WordPress Multisite installation and I am trying to configure the plugin to run in ‘Mutual Account, Mutual Access Key’ mode so that all 5 sites within the Multisite network can use the same CleanTalk Access Key. This makes sense as all sites on the network belong to the same client.

However, whenever I try and save or update the Access Key in the plugin, to match the Access Key generated within our CleanTalk account, the update fails and the plugin holds on to the old Access Key that we have now removed from our CleanTalk account.

Obviously, without a valid Access Key the plugin is not working on any of the sites within the Multisite network.