When a file is loaded UltraEdit checks the first 3K of the file and if it contains more than one NULL (HEX 00) the file is considered a binary/HEX file. So check, if your log files contains bytes smaller HEX 20 (space) which are not carriage return (HEX 0D), line feed (HEX 0A) or tabs (HEX 09).
If it is possible modify the log file creating program to not write binary bytes to the log file.
It is possible in UltraEdit to open files with HEX 00 in ASCII mode. You have to activate in Advanced - Configuration - General at the last section Advanced/Problem Solving the option Allow editing of text files with HEX 00's without converting them to spaces.
If you activate this option, read also the help about it, because working in this special mode is very critical.
Best regards from an UC/UE/UES for Windows user from Austria