Thanks for bringing this up to us, @AirKevin.

We have an investigation where some of our users getting the Unexpected problem: -30006 when applying a payment to an invoice. Our product engineers are aware of this problem and are now working hard to fix this the soonest.

I recommend giving our Customer Care Team a call to add your account details to the list of affected users. This way, you'll be updated as soon as a fix is implemented. Here's how:

1. Click the Help menu in the upper-right hand corner and click Talk to a human.
2. Type in "Talk to a human", then press Enter.
3. Look for I still need a human and click on it.
4. Select Get help from a human.
5. Choose between Chat with us, Schedule an appointment, or Have us call you.

 
Please check out our support hours to ensure that we address your concerns on time.

If you have further questions or concerns, feel free to reach back out anytime. Stay safe.