Attr change run failed in PC

Hi Experts, One of my process chain was failed due to Attribute change run and the message is showing Master data extractor running. Change run not possible, we design process chain like below

Start ---->DTP for MD-----> Attribute change run

here my load was completed successfully then how can show this error message

Master data extractor running. Change run not possible?

at the same time one more attribute change run is running in my system? is this effected? and can't we run 2 att change runs at a time?

Regard - Narendra