I had the same problem this morning. Tried to log in with the app and got the server error 'handshake failed' message. I am running the original V1 Texecom connect app on my Samsung S10e phone (Android 12). App was working fine yesterday morning. The app works fine on my wife's iPhone and on my old Honor 9 smartphone (Android 9). Perhaps something to do with Google apps on my Samsung updating overnight?
I tried deleting the app but when I came to reinstall from Google Play Store, only the Texecom Connect V2 app is available. So I registered and entered the activation code, but it then asked me to enter an Installer ID, which I don't have (never needed with the V1 app). I will need to contact my installer tomorrow.