When I first saw PHP 3.1, and was
@overseers is correct.
ionCube doesn't support PHP 8.3.x, and there seems to be a bug with mbstring.
If you build PHP-FPM 8.3.x without those 2 options you will be good to go.
IF you need the mbstring option for something, then you will have to stay with 8.2.x temporarily.