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uC/USB CDC ACM Serial fails at 3 or more ports

I'm creating an application which needs up to four virtual COM ports from the USB on BF707. This application is using uC/OS + uC/USB CDC + uC/USB ACM Serial. I can only get up to 2 virtual COM ports working properly, the 3rd one hangs on data RX and the 4th one crashes in the DMA handler on connection. I traced the crash to this location: \usb\controller\musbmhdrc\adi_usb_dev_musbmhdrc_intrpt.c line 127. It looks like pEpCurURB is NULL.

I attached a sample application which reproduces the problem on Windows 10 (I haven't tested other versions).