ispconfig_update.sh script fails after downloading. Code: >> Update PHP Parse error: syntax error, unexpected '[' in /tmp/update_stable.sh.i6K9DC5Hk1/ispconfig3_install/install/lib/installer_base.lib.php on line 296
Yes. 5.3.3 Can't upgrade OS because of VPS/kernel limitations. I do have newer PHP versions installed, but the system version is outdated.
If the OS version is too old and not supported by ISPConfig anymore, then config file templates that ship with ISPConfig might be incompatible and also the default PHP version of the OS might be too old as in your case, it's better to migrate the websites to a new system then if you can't update the current OS and keep the ISPConfig version as it is until then.