Channing Tatum and Jenna Dewan's divorce has turned nasty ...  all over child support, custody and allegations of FaceTime sabotage.

Our Jenna sources say she was stunned and super upset when Channing filed legal docs Tuesday asking a judge to come up with a holiday custody schedule.  Our sources call it grandstanding, because Jenna had already signed off on the entire schedule.  Our Channing sources counter Jenna agreed to some of the schedule but not all of it ... thus the need for the judge.

But that's just the beginning.  Several sources say, although Channing and Jenna had a custody schedule for 6-year-old Everly, there were times Jenna was working and asked to change the schedule but Channing refused.

Child support has also become a bone of contention.  Our Jenna sources say Channing isn't contributing, but our Channing sources say there's a very specific, joint account with community funds that cover Everly's expenses.

And yet another issue -- communication.  We're told when one parent has Everly, that parent has to allow the other parent to FaceTime with the girl everyday.  So Jenna sources say Channing doesn't allow her to FaceTime.  They also claim when Jenna has the child Channing almost never FaceTime's with her.

Our Channing sources call BS ... saying Channing does indeed FaceTime with Everly when she's with Jenna, but they say Jenna's so difficult Channing just FaceTime's through the nanny.

Jenna's rep, Nicole Perna, tells us, "Jenna's priority has always been, and continues to be, her daughter's well-being."

Like we said ... lots of issues. But, hey, at least they're both legally single, right?