The 3 reasons your first layer fails (and the 2-minute check)
If the first layer looks like spaghetti, it's almost never the STL. It's one of three things:
Bed isn't actually clean. IPA isn't enough if you ever touched it with fingers. Wash with dish soap, dry, then print.
Z-offset is 0.1mm too high. Paper test should drag, not slide. If you can pull the paper out with no resistance, drop Z.
First-layer speed is still at 50mm/s. Cut it to 20. Most "bad files" start printing the second you slow that down.
Do those three before you re-slice. If it still fails after that, the machine has a real fault — that's the hour we spend on Live Fix.