Personally I tell people that IE is not recommended at all. I've always hated dealing with it, and apparently so has microsoft, since they announced that it would be cut completely.
Technically IE 11.x is supported, try another browser though (like Chrome), and see if they can open the database. If they can open it in another browser, than it's probably some buried setting that's preventing them from using IE.
Also, provide more details about what will not work, can they open the webdirect home page and it's just the database that will not open? does it render a blank page? what URL is in the address bar? screenshots? etc..
Thank you for all kind replies.
I solved this problem with my network engineer.
I was using FM Server via VPN. (static IP)
Well-working computer is the same network with VPN,
the other computer had different IP but its IP came from the same block with VPN. (in a same building)
So, the computer couldn't find VPN's IP.
The engineer make the computer's IP roundabout, it works!
ps. sorry for my poor English. I'm not a English speaker.