@goody2shoos I’m sorry that you have lost work.
To be honest there has been similar reports, but is not related to the Nightly Build, it appears to be related to how Pencil2D manages memory. It seems there’s an uber critical bug that flushes all the drawings and the file away corrupting the files and making them unusable, however this has happened only to people that have files with more than 200 frames and were using PCL files (which come with the external folder by default)
To clear some doubts the FAQ only says that “PCL” files create the folders, not PCLX. Newer Pencil2D has two filetypes, legacy files with are PCL, and new type PCLX. I’ll try to change it over the FAQ to make it clearer later though.
PCLX files store the bitmap / vector / sound information inside the file as opposed to an external folder. The downside to this file seems to be that the more images, frames or layers you have, the bigger the file will be, so if you had a lot of frames in the file, that might explain why the app is crashing before you can open it.
In summary, backup your Pencil files (either PCL or PCLX) often in different places, who knows what sort of wacky behaviour will come up next. Sigh.
The only thing I can ask of you is to send over the file for closer inspection, you could use a file upload service like mega.nz or if you have a google drive or dropbox account upload them there and share a public link so I can test it. Any findings will be reported to the developers, however I don’t really know if this is something that can be fixed as easily.
I’ll be pending your response