I've been using Premiere Elements for a few years now with no problems. Just went to use it again yesterday and it is having problems with .mp4 files. Import my mp4 file per usual and it shows up unclear with green lines across it, making it impossible to work with (see screen shot).
The file looks fine in the project assets window, and exports cleanly. And you can view the file in its original format and after exporting using other software -- you just cannot see it properly to edit.
Am using Premiere Elements 2018 with Windows 10 operating system. After an hour-long online chat with support, they finally tell me they don't support that version anymore. So I upgraded to 2021 and it's still an issue.
The green lines mean you have a video driver issue. The proable cause was that an update was done to your Windows system or your GPU software. The solution is to make another update to the most current or, roll back the video driver to an older, more compatable version.
Fixing the video driver will fix it for both the 2018 and 2021 versions. If you don't want the new version, bought it from Adobe and did so recently, you can get for a refund.