I have come across this bug a few times now, and it appears to happen because the allotted ram for JAVA is full. In my case, due to loading all the mods i have selected to run with minecraft. The workaround i have found is on the launcher screen, select a previous version of minecraft (when 1.11 was out i used 1.10, now 1.12 is out, so i use 1.11) and load in minecraft with no mods at all. Join the server again and you will be functional again and able to move between portals.
Jared LaFont
I have come across this bug a few times now, and it appears to happen because the allotted ram for JAVA is full. In my case, due to loading all the mods i have selected to run with minecraft. The workaround i have found is on the launcher screen, select a previous version of minecraft (when 1.11 was out i used 1.10, now 1.12 is out, so i use 1.11) and load in minecraft with no mods at all. Join the server again and you will be functional again and able to move between portals.