The Japanese conglomerate "determined that there were facts that could be classified as non-public material facts under insider trading regulation," it said in the filing.
The group announced a major cut in its stake in wireless unit SoftBank Corp on Aug. 28, followed by the sale of chip designer Arm to Nvidia Corp on Sept. 14.
A company spokeswoman declined to comment on whether SoftBank is again in a position to buy back shares.
Buybacks have been a major driver for SoftBank's shares, which climbed 160% from March lows to peak on Aug. 3 when they closed at 6,932 yen.
The shares have since fallen around 8%, and closed flat on Tuesday.
SoftBank previously signalled a slowdown in the pace of its remaining 1.5 trillion yen worth of planned stock repurchases.
The group has been selling assets to fund the buybacks and raise cash. SoftBank has invested in publicly traded tech stocks and, sources told Reuters, in equity derivatives.
Executives are frustrated with SoftBank's share performance and have held early stage talks on taking the group private, a person familiar with the matter told Reuters.
By Sam Nussey
