Is an “ex parte communication” illegal?
Ex parte communications are only illegal if they involve substantive issues. The judges were discussing scheduling issues; since the D.C. case has been stayed due to President Trump’s appeal based on presidential immunity, there is no scheduling conflict which would prevent the Manhattan case going to trial on March 25.
I’m uncomfortable with the word “illegal” if you read into that that it’s crime. It a violation, and a serious violation, of judicial ethics codes