how to trace flatboard when a reply has broken it

[gritd]

Flatboard destroyed itself after a few weeks of use by now showing me this message:

Warning: Undefined array key "reply" in [forum url]/index.php on line 95

Fatal error: Uncaught TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in [forum url]/index.php:95 Stack trace: #0 {main} thrown in [forum url]/index.php on line 95

I have no idea where ot comes from (perhaps a bot/hack reply) nor how to tacke this and I needed to deactivate the forum. First thought was to check reply entries but .. how is this built up?