I tried it with latest stable version of Pro/Enterprise Version.
Sadly it doesn't work there either.
Observium 22.8.12241
Software versions
o OS Linux 5.15.0-48-generic [amd64] (Ubuntu 22.04)
o Apache 2.4.52
o PHP 8.1.2 (OPcache: DISABLED)
o Python 3.10.6
o MySQL 8.0.30-0ubuntu0.22.04.1 (extension: mysqli 8.1.2)
o SNMP NET-SNMP 5.9.1
o RRDtool 1.7.2
o Fping 5.1 (IPv4 and IPv6)
Memory Limit
o PHP Unlimited
MySQL mode
o MySQL NO_ENGINE_SUBSTITUTION
Charset info
o PHP UTF-8
o MySQL utf8mb3
Timezones info
o Date Tuesday, 27-Sep-22 15:52:05 CEST
o PHP +02:00
o MySQL +02:00
Confirmed issue with php 8.1.
Fixed since r12314.
(Will released in next CE)