For some reason, ProConnect dropped my firm's phone number. After I (re)entered the phone number (at Firm Information), I changed (on the tax return) the preparer to "Select Preparer", then I ran Check Return (just to force the program to save data). After that, I re-selected myself as Preparer again. Afterwards, I ran diagnostics again (out of habit), e-filed the return, and it went through!