Iphone footage 4k + 1080p + 720p / 29,98fps x 119,97fps x 239,99fps = 25fps

Hello,

My question is : What is the optimum way of handling various qualities of footage that have different fps and quality?

What i have been doing so far :

I choose the iphone 120fps footage (as it is 1080p), i click right, assume frame rate as 25fps

I create a timeline of which the sequence matches the footage

i then drag the 4k files into it, (and change the scale so it isn't cropped)

I edit my footage without any problems

I render the sequence in to out, it all reads fine

I save and close

when i re-open the project, i get : low level exepection error 37

I delete all cache files, render files, preview files

I reopen the project, it works fines, i render the files, it works fine ...

i save and when i re-open it's not working again...

I can only ever do it once, why do i need to start from scratch ?

is the problem the render preview files ? but i'v tried all of the formats offered, mpeg, h264 and the other one

So where am i going wrong please ? and please let me know if i need to provide more crucial information ?

lots of love,

Louis