http://www.the-digital-picture.com/News/News-Post.aspx?News=14935

По ссылке - статья о несовместимости режима Live View у Canon EOS 760D и EOS 750D с данным объективом и другими стеклами от Сигма. Указаны метод борьбы (бесплатная перепрошивка в сервисе) и серийные номера объективов, которым это поможет.

Сам попал в такую ситуацию. Успешно разрешилась апдейтом прошивки объектива.

From Sigma:

Notice to our Canon mount lens users: Thank you for purchasing and using our products. We have found that Live View Mode cannot be used on the EOS 8000D (Rebel T6s / 760D) and EOS Kiss X8i (Rebel T6i / 750D) released on April 17th 2015 by Canon Inc. with some of SIGMA’s Canon mount interchangeable lenses. For those who own the lenses, we are going to provide a firmware update free of charge.

Phenomenon: When the Live View Mode button on the camera body is pressed, the operation stops without displaying images on the rear LCD monitor.

Products and Serial Numbers that require the firmware update.

Current models:
17-50mm F2.8 EX DC OS HSM No. 12651501 or later
18-250mm F3.5-6.3 DC MACRO OS HSM No. 13044001 or later
APO 70-200mm F2.8 EX DG OS HSM No. 12713001 or later
APO 50-500mm F4.5-6.3 DG OS HSM No. 12902001 or later
120-300mm F2.8 DG OS HSM | Sports No. 50064494~ 50347793
APO 150-500mm F5-6.3 DG OS HSM No. 12669751 or later

Discontinued models:
17-70mm F2.8-4 DC MACRO OS HSM No. 12665001 or later
18-200mm F3.5-6.3?DC OS HSM No. 12851001 or later
18-250mm F3.5-6.3 DC OS HSM No. 12656101 or later
APO 50-150mmF2.8 EX DC OS HSM No. 12839001 or later
APO 120-300mm F2.8 EX DG OS HSM No. 12676440 or later
APO 120-400mm F4.5-5.6 DG OS HSM No. 12971601 or later

* For lenses that have been repaired in the past, they may require the firmware update even when the serial number is not listed above. In case the mentioned camera does not display images on the rear LCD monitor when Live View Mode button on the camera body is pressed, the firmware update is necessary.

We will make a further announcement when the firmware update becomes available.

For those who own the SIGMA USB DOCK, it will be possible to update the firmware of 120-300mm F2.8 DG OS HSM | Sports through SIGMA Optimization Pro. We will also announce its availability when the update is available.

* If you use the SIGMA USB DOCK with 120-300mm F2.8 DG OS HSM | Sports, even when the lens does not require the update, its firmware will be updated to the new version.

* For those who use APO TELE CONVERTER 1.4x EX DG or APO TELE CONVERTER 2x EX DG with the 120-300mm F2.8 DG OS HSM | Sports, please contact your nearest authorized subsidiary / distributor of SIGMA for further support.