The JIT stores its compiled code in the data segment of the binary memory image. On PaX (and GRSecurity) this is a security breach.
Solution #1: Disable PAX on hlds (chpax).
Solution #2: Disable GRSecurity and PAX.
Solution #3: Use a safe binary. The nightly builds will contain "amxmodx_mm_i386-safe.so" from now on, this will be "PaX/GRSecurity" safe (compiled with assembler core, not C or JIT core).