The 9/11 Commission Report published July 22, 2004 would be the goto source for any such research. The exhaustive bipartisan 450 page report detailed who knew what and when they knew it in excruciating detail.
Below is what I have found.
On page 262, August 6 2001, It says:
"the President's daily briefing contained the words the "CIA and FBI
are investigating a call to our Embassy in the UAE in May saying that
a group of Bin Ladin supporters was in the US planning attacks with
It goes on to say that the FBI had a group of 30 agents working on 70 ongoing bin Ladin- related investigations.
Also from page 262:
Late in August 2001, a foreign service reported that Abu Zubaydah was considering mounting terrorist attacks in the United States after
postponing possible operations in Europe. No targets, timing or
method of attack were provided.
And those were the last "warnings" discussed by high level Bush Administration officials of an impending attack until after Sept 11 2001.
Also to support your premise that a Taliban Foreign Service official in August of 2001 would have warned the United States, The report details a rift between the leader of the Taliban and Osama in the months leading up to the attack on whether to attack the US or not.
On page 251 the report says:
"There is evidence that Mullah Omar (the leader of the Taliban)
initially opposed a major al Qaeda operation directly against the
United States in 2001. By July, with word spreading of a coming
attack a schism emerged among the senior leadership of al Qaeda.
Several senior member reportedly agreed with Mullah Omar. (against an
attack).... those said to have opposed were mighty figures in the
organization. including Abu Hafs the Mauritanian, Sheikh Saeed al
Masri, and Sayf al Adl."
Also from page 252:
According to KSM, in late August, when the operation was fully
planned, Bin Ladin formally notified the al Qaeda Shura Council that a
major attack against the United States would take place in the coming
weeks. When some council members objected, Bin Ladin countered that
Mullah Omar lacked authority to prevent al Qaeda from conducting jihad
outside of Afghanistan.