This problem mainly occurs when using Seamless Transitions “Legacy”.
To avoid this error, use the special version of Seamless Transitions “MOGRT”.
The main advantages of the MOGRT version:
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No crashes. MOGRTs do not crash Premiere Pro, unlike native transitions.
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More transitions. Without Premiere Pro restrictions we’ve added 200+ transitions.
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Flexibility. Now you can customize the transitions.
Please check this video tutorial for using ST MOGRT: Seamless Transitions Tutorial Updated (MOGRT) - YouTube