There could be a few different reasons that Lark ends a conversation right after opening the app.
Please refer to this article first to see if the troubleshooting steps alleviate your issue first:
Read more about the potential causes below:
Weak Data Signal
Lark's chats and calculations of your data require access to the internet, so if you are in an area with a weak signal, it can take too long for your phone to communicate with our servers. As soon as you get back to an area with a stronger signal, Lark should resume chatting right away.
While we are hard at work preventing any server downtime, from time to time it can be possible, and a common symptom is that Lark will stop chatting.
In some cases, it has been reported that the Lark chat screen will not show any chats at all. Additionally, the entire app becomes completely unusable and appears locked down or stuck. If this is the case, please turn off your iPhone by holding down the power button, and sliding to power off. After your iPhone has powered down completely, you can restart it, and open Lark again. It should be functioning properly. (We've reported this bug to Apple, and it should be fixed in a future iOS update.)
If you double press the Home Button, it will bring up the Multitasking (or fast app switching) view. Swipe left until you find the Lark app icon and screen, then swipe up to "force quit" the app. This will force the app to refresh the next time you open it.
If that does not work, try restarting your iPhone by holding down the power button until a slider appears at the top of the screen. Turn off your phone, then after the screen powers down and the spinning icon disappears, you can turn it back on using the power button.
In rare cases, due to constant app crashing, it may be appropriate to delete the Lark app and reinstall it from the App Store (not from a backup on your computer or iCloud Backups). Note: keep your Health data, when the on-screen alerts ask you. It's easy to accidentally remove your Lark data from Apple's Health app.