PixInsight does not set any specific limit on the maximum number of characters that can be used in a file or directory name. As Wouter has said (in the other thread), the limits in this regard are always imposed by the host operating system.
On Windows, the maximum length of a full file path (which includes the drive specifier, the directory path, the file name and extension) is 260 characters. Starting from Windows 10 version 1607, this limitation has been removed, but you must opt-in to the new behavior, which remains disabled by default. See
this Microsoft document for complete information.
Your issue is that the file name contains an invalid file name character, the ' ' (space) character.
PixInsight fully supports spaces in file and directory names, on all supported platforms, so this should not be the problem.
On a side note, I really cannot understand why the people insist in using file names like this:
"Flat_filter7_2020-02-01_060400_1x1_0.29sec_-15CFlats ProED ASI1600 Pro 20191228_fpos_18941_amb_11.0C.fit"
IMHO, such file names are completely unmanageable. Image file formats provide standardized mechanisms to store metadata describing filters, acquisition dates, binning factors, exposure times, frame types, temperatures, cameras, telescopes, focuser positions, astronomical objects and coordinates, among many other items. There is no need to generate absurdly long and complex file names, which are just an excellent way to call for problems. One should use well-structured directory trees to keep the data perfectly organized, taking care to store all of the required metadata in each raw frame. A file name such as "001.fits" is an excellent option.
