XF 1.5 PHP 7.4 error "trying to access array offset on a value of type bool"

[Cassandra]

I upgraded to PHP 7.4 and since then I'm getting the following server error:

I see high CPU usage as well.

Should I downgrade to PHP 7.2?

 

[021]

.XF1.5 no longer supported. Use an old version of PHP or upgrade to XenForo 2.1

XenForo 1.5 End of Life Schedule

With XenForo 2.1 now in beta testing, and a year having passed since XenForo 2.0 became our stable, supported release, it is time to talk about our ongoing support for XenForo 1.5. XenForo 1.5 is now considered a legacy product and is no longer in active development. We will continue to support...

xenforo.com