Update:
I've done some more research. I haven't been able to trigger the transition cutting to black (TCB) error on the project I previously posted a link to on the board. However, by experimenting some with different projects and watching the cache folder I've discovered some things.
Cache files are generated in three ways....
Process 1: Some cache files are generated automatically when the project is loaded. These are video index files generated from the original video files included in the project. They are generated while the "Laoding Project" dialog displays.
Process 2: After the "Loading Project" dialog finishes, additional audio, video, and image cache files continue to be generated in a background thread. This background thread may go on creating additional cache files for several minutes if the project is large, but the number of cache files will stop growing at some point if the program is left idle. I assume the cache files generated by this background thread are used for generating previews.
Process 3: When a segment is being exported, additional cache files are generated. I assume these additional cache files are needed for the final output because the final output is generated in a higher resolution than the preview.
Attempting to create an export file with TCB errors, I performed the following experiment on three different projects...
1.) Uncheck "CLEAR UNUSED CACHE FILES WHEN CACHE SIZE EXCEEDS (MB)..." box, check "CLEAR ALL CACHE FILES ON EXIT" box.
2.) In the Windows file explorer, go to .../Users/<user_name>/AppData/Local/Temp. The file VideoPadCache appears here.
3.) Exit VideoPad then re-open with the selected project.
4.) Let VideoPad sit idle and watch VideoPadCache in the file explorer. Wait until the size of the folder stops growing (end of Process 2). Keep refreshing and right-clicking on the folder to check the "PROPERTIES" dialog. If the neither the number of files in the folder nor the folder size changes for a period of 2-3 minutes, Process 2 has ended. Record the size of the cache.
5.) Go back to VideoPad, check "CLEAR UNUSED CACHE FILES WHEN CACHE SIZE EXCEEDS (MB)..." and set the limit to roughly half the cache size recorded at the end of Process 2. For example, when the cache size was 2.2 GB, I set the limit to 1000 MB (1 GB).
6.) Exit VideoPad and re-open the project once more.
7.) This time don't wait for Process 2 to finish. Immediately export to video.
I notice there is some element of chance, but following the steps above increases the likelyhood of seeing TCB errors in the exported file. In my case, TCB errors seem to occur exclusively after still images with a pan-and-zoom effect added. I was also only able to get them to occur in projects with nested sequences (i.e. one main sequence consisting of two or more subsequences placed end-to-end). They also seemed much more likely when the size of the project was large (2 GB or more) with many panning/zooming image clips.
While watching the VideoPadCache folder, I noticed that the total size of the folder always manages to exceed the limit set next to the "CLEAR UNUSED CACHE FILES WHEN CACHE SIZE EXCEEDS (MB)..." checkbox option, yet once it exceeds the limit it occasionally shrinks by a small amount before growing again. From my observations, the threshold condition for deleting "unused cache files" is being triggered by the thread in Process 2, not the thread involved in Process 3. Maybe the criteria for determining and communicating whether a cache file is still in use is faulty in some cases.