Hi @BerndD as described here FP4.SREL.15.14.4 is indeed only a security patch update for users who are still on FP4.QREL.15.14.3 (Android 15) & FP4.TP31.C.0143 (Android 13).
For users who are on FP4.QREL.15.15.2 they are indeed not able to install FP4.SREL.15.14.4.
As shared previously, we are aware of the issues that some Fairphone 4 users on Android 15 are experiencing, and the relevant teams are actively working on fixes. 