SL/SE1 forearm pain

There are few threads on this. I experience acute pain in the flexor carpi ulnaris, right in the middle of the forearm. According to anatomy pictures that is where it seems to turns into tendon/connective tissue. This pain comes when I try to be square, shoulders parallel to ground, and protract as much as possible. The pain has not been present when approaching the exercise without being square. The pain effects basically all other exercises in a negative way. Are there more people experiencing this issue?
Without weight I can move the hand freely, but when I do a bicep curl motion with a weighted object the pain is being felt.

can you post a photo of your grip?

also are you following the handstand one program? the wrist series is pretty useful for beginner to build up some ulnar deviation strength.