PO 7.5: String index out of range: -1 on IDoc AAE receiver

Dear experts,

I have numerous IDoc AAE receiver channels in PO 7.5. One of them has this strange error "String index out of range: -1"It does receive messages (ORDERS05 IDocs) and processes them correctly into SAP ERP, no errors at all.

Has anyone seen this before? The log says:

No other IDoc AAE Receiver channel shows that error. I have the same partner/Business system/channel in our TEST system and it does not have that error. I compared channel settings to others of the same type and no difference.
I checked with xpi_inspector but do not get more information.
I already created a new channel for the same partner/Business system with a different name and it shows the same
error immediately:

I appreciate any idea on this. I wonder if a system restart would solve this. It does not bother me really because there are no issues but something seems to be off.

Best regards,
Peter