Please update your #matrix clients: https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharing
If you are using versions from https://f-droid.org, I recommend to switch to a different one, at least until they catch up with building the fixed versions (it's unclear how long that will take. Might be one or multiple days).
An updated version of #Schildichat can instead be installed from the author's #FDroid repo here: https://schildi.chat/android/install-from-sc-fdroid/
Updated (FOSS) versions of #ElementAndroid are currently not yet available.