MP4 missing after reopening project

Working on my current project to following issue has occured: When saving and closing the project the video files (MP4) imported in most - but not all - slides are replaced my a miniature version of slide, forcing me to revisit all of the replaced files.
This happens every time I close the project. Which files are replaced seems to be random, but it usually happens on about 80% of the slides. Renaming the project or chaning the data path did not solve the problem.
Thank you in advance!

ActivePresenter Version: 7.5.5

OS: Win 7



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