Can I file an additional amendment if the current one has already been accepted, but not yet processed?

It is taking the IRS six months or longer to process amended returns.   Putting yet another amended return into the system would just confuse the IRS and cause longer delays.   

Part III Explanation is the place where you make sure the IRS is not confused.

What kind of error?

Did you submit the supporting forms and schedules as required?

If so, and there is an error IRS may discover it and adjust your amendment as necessary for you, or contact you for more information.

Expect to wait months for your 1040-X to go through.