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:
No crashes. MOGRTs do not crash Premiere Pro, unlike native transitions.
More transitions. Without Premiere Pro restrictions we’ve added 200+ transitions.
Flexibility. Now you can customize the transitions.
Please check this video tutorial for using ST MOGRT: Seamless Transitions Tutorial Updated (MOGRT) - YouTube