Uncaught Error in utils.php:136

Hi,
I have encountered an Uncaught Error pointing to your plugin yesterday. I have upgraded the plugin to 1.2.7 recently. The problem occurs when I try to change the status of an order. The changes is made between two status I have created (Received and Ready). I use your plugin’s mail template for status Processing by adding my content with the “Other plugin information” block. It used to work perfectly.

I can reproduce the problem. I can stage my site if this could be useful for investigation.
I have added a the complete error log.

Thanks,
Bernard