Short: IP v1.5.10 is not compatible to PHP 7.4!
Its compatible for PHP 7.0-7.3
But to answer your question:
As some events did get canceled (because of COVID-19) I do not have to go abroad and can release v1.5.11 very soon. It will support PHP 7.0-7.4 so in v1.5.11 PHP 7.4 support will be added.
But have to have some internal talks with the other two maintainer if there is anything that blocks the release as all of us have a veto right if there is a plausible reason.
Yes there is already a Demo and a internal Demo, but pls just wait some days. And for the meantime just switch back to PHP 7.3