If Silent Phone is open on both sides of the conversation but you are unable to exchange messages with your partner then new encryption keys need to be made. The best and fastest way to achieve this is by deleting the current conversation and starting a new one. Please note that this will erase the content of the conversation (Android users might want to save the conversation before deleting it).
For Android and iOS users, on the main conversation window, simply swipe right on the conversation you wish to delete and tap on the trash can located on the right side of the screen.
The user may also contact the Support Team with the actual error message and allow us to trouble shoot and assist.