Hi Rafal,
This is an issue with a recent Android System WebView update that causes a visual glitch in our app. There are two solutions:
1. Change the message editor in Settings > Composing and Sending > New message editor (this is enabled by default, disable it).
OR
2. Open the Play Store > Find the WebView app > Tap Uninstall to remove the most recent update. Don't worry, this does not fully uninstall the WebView app/service.
Martin