Android has a limitation for security reasons, it only allows you to install the very same version of an app for the other users.
I guess you’ve updated F-Droid, but the main blue button still serves older version 1.14.
Try to install latest 1.15.2: https://f-droid.org/repo/org.fdroid.fdroid_1015052.apk
ref: Bump F-Droid apk link to 1.15.4 (!823) · Merge requests · F-Droid / Website · GitLab