After XtraBackup full-backup the sys_exec UDF fails

After a full backup with the innobackupex script with the options --compress --compress-threads=4 --parallel=4

using xtrabackup 2.1.4 64 x86_64 on a MariaDb 5.5.32

The sys_exec() UDF always returns -1 and does not execute the command between the brackets (https://github.com/mysqludf/lib_mysqludf_udf#readme)

The function is still registered

recreating the function with drop/create shows no errors but doesn’t help

The logs show nothing, restarting the db does fix the problem

Does anyone know how to fix this?