In folder 'www' I have 1699 items. 8 of them are subfolders. In the subfolder of 'www', folder 'js' has 109 items

Most of the time it happens on www folders in apps. Sometimes this folder is opened. Then the problem accurs on the js folder in the www folder. Maybe it is timing, or the amount of files and subfolders.

I hope this helps.

Let me tell you I used Textastic very long with much fun and I hope you can fix this problem

Yes, Dropbox is linked. The problem appears on some folders, not all.