Same problem occuring on our server right after upgrading to 1.14.3 Pre-Release 1. We are also using valinnatweaks.
edit: went through all the datapacks and found the culprit: double shulker shells datapack from vanillatweaks.com causes this mess.
"1.14.2 or later". I really hope it's not later. This is a serious bug that ruins servers because the lag it causes. It can literally make the game unplayable. This shoud be one of the most critical bugs to fix.
It did works perfectly fine with default controls. It was not a keyboard issue. But that made me think it still might have been nonminecraft issue and it was. Found out that Microsoft decided to add a short key combo ctrl + space bar to their powertools without asking. Disabled that and it's works again. Sorry for the false report.
https://answers.microsoft.com/en-us/windows/forum/all/ctrl-space-not-working-while-pressed-together/f6ef4fae-5403-429a-8c8d-484095feee45