What is on line 56?
Doh! Sorry, still half awake. Line 56 is where the assert was raised in the source code Code says
if( !fgets( line, sizeof( line ), fp )
// first line starts with "ACCEL_ASCII" with optional stuff on same line after that.
|| memcmp( line, ACCEL_ASCII_KEYWORD, sizeof(ACCEL_ASCII_KEYWORD)-1 ) )
THROW_IO_ERROR( "Unknown file type" );
So basically the file does not start with "ACCEL_ASCII ...". The file might not be ASCII format, or is not expected format. I think to investigate further I would need to see the file.
If you can try the latest nightly build, the PCAD import might work. I do not recommend nightly builds for "production" use.
Also, the PCAD importer was originally part of https://sourceforge.net/projects/pcad2kicad/, I don't know if that is still maintained, possibly it can be run standalone.