Gridbox Error: Unsupported operand types: string / int

Using a really old version of Gridbox generated the following problem by opening Gridbox from BackEnd:
Error: There is an Error
0 - Unsupported operand types: string / int
And I'm only able to go back to the control center
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Gridbox Administrator Version 1.2.10 (06 Oct 2015)
Gridbox Core Site Template Version 1.2.9 (06 Oct 2015)
PHP 8.0.30 / Joomla 3.10.12 Stable
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Yes, I know that this page has to be replace by an newer version - but ...
Regards Heinz

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