Hi Ziga
Let's start with the basic info requested in the FAQ
viewtopic.php?f=21&t=90190 -
• a link to the log file,
• a description of the timeline and clip (e.g. what codec is it, what effects it has etc),
• a description of your render settings (e.g. are you rendering locally? what codec settings? what render settings apart from individual clips?)
• your system - which mac, what hardware specs, what GPU processing mode? (please note that the minimum requirements for 16.1.2 include macOS Mojave)
To rule out issues with project or render settings, try isolating the issue using steps similar to these:
• copy clip1 and clip2.
• In the project manager, context click, enable dynamic switching, create a new project and paste these into a new timeline.
• Using individual clips and default render settings, render to a folder on the Desktop as a ProRes clip.
• you can use in-out points to isolate specific parts of the clip for a quicker render test
• once you can establish that it renders locally, you can tweak the settings to determine the difference in settings causes the issue