It’s hard to say exactly what the issue is without taking a deeper look at your account. The INVALID_RECIPIENT error code description is “Recipient internal id does not match an existing entity.”, meaning that you’re trying to email someone and it’s not matching an existing employee/vendor/customer/contact record with an existing email.
Upon first glance I would think that this is an error from a custom script/workflow in your account. My first step would be in sandbox where you can replicate the issue to go to Customization > Scripting > Scripted Records and view the “Journal Entry” record there. Review this page (and all the tabs) for anything that looks like it might even remotely be related to this issue. Then one by one disable those scripts/workflows and attempt to enter the journal entry and see if you get the same error. Hopefully when you disable a specific one the error will resolve/disappear. Then you’ll know what script/workflow is causing the issue and you can dig a little deeper into what is actually happening in it that is causing the error.