EOS R5 Mark II won't take a photo with a Rokinon 14mm lens

[shawl]

I am trying to use a Rokinon 14mm f/2.8 ED AS IF UMC with AE chip lens on my Canon R5 Mark ii that was purchased new in 2018 for use with a Canon 5diii. (The lens is still available and, from the B&H site, is NOT recommended for the R5ii!) Lens is manual focus without stabilization. Lens is fine with my Canon 5d mark iv. Proper Canon adapter is used. When the shutter is depressed, it seems via sound that an image has been taken; the red recording light goes on but nothing happens...camera is locked up until the battery is removed to reset.

I have tried a number of things: release shutter w/o lens turned ON; when I checked to be sure the focus mode was manual, it was. When I checked to be sure image stabilization was off, it was.

None of this allowed a picture to be taken.

Perhaps nothing can be done, but if anyone has further suggestions, that would be appreciated.

[dannygirl]

I also have the fully manual Rokinon 2.8 14mm, previously used successfully on Canon 6D exclusively for astro, now wanting to use on R6mkii - and shutter won't fire.  

I've engaged the 'release shutter without lens', and this seems to work?  Aperture is set manually on the lens, so will be 2.8 (if set), despite camera showing F00.  

This probably won't work for manual focus lenses which does not have manual aperture ring also - but hope this helps at least SOMEbody out there.