Hey @marca, I know this is going to sound weird, but is there any chance that there’s an insanely low character limit on the plugin preview function?
Yesterday, I submitted bug #7300, and I have been banging my head against the wall thinking I had a logic or syntax error, but what I’ve learned (after literally hours of troubleshooting) is that reducing the size of the function body resolved the issue! 
The preview function of the plugin exhibiting this issue is less than 3K, but when I delete less than 1K of code (or just comments!) - i.e. simply reduce the size of the function body, then it works!
Is it possible that the amount of code allowed in the preview function body is off by a factor of 1000 or so? 