This error is from a non-standard Apache module
mod-qos, this has custom modules that put restrictions on your hosting environment; unfortunately where we've been updating code in every new version to optimize the overall codebase, modules like this will over zealously restrict resources depending on its rules.
UBB.threads is a basic PHP script that assumes a standard set of web server modules are present, we can't really account for resource usage from popular sites on servers with such restrictions (most ask users to move to higher services based on how much persistent system resources they use).
I can however tell you that every single one of my sites is on Cloudflare on a free plan.
