Render Dots Have Turned to a Solid Red Line

Hey, I've been using FCPX for years, however recently upgraded to MacOS Big Sur and have since been having a few issues with rendering in my timeline.

I duplicated a library as I had to edit a new episode using the previous library/project/timeline as a template. I'd done this for previous episodes and all worked well, however when taking the same actions for the latest episode, I left the timeline to render, it rendered half the timeline and then turn the render dots to a solid red line as if the sequence was corrupt(?). When trying to play this section, the audio plays however the picture is stuck on whatever frame you first played; it stays this way until you pause, and then it reveals the frame you have paused on.

However... the sequence seems to play as normal when I disable the two text boxes and one of the 'solid colour' bars I've inserted along the bottom of the screen - but I need these elements so can't leave them disabled!!

Does anyone please know how to fix this? I've searched everywhere but can't find anything which directly addresses the problem!!

(Another related thing, which I've learnt to deal with after years, (however I don't think it's a normal function of FCPX) is that my timelines have to render every time I re-open a Library, Event or Project - even if the completely rendered just before I last closed them down. Surely Final Cut should hold onto these render files and not have to re-render every elements on launch, EVERY TIME!?)

I'd really appreciate anyone's help. Thank you all.