You might have generated more systems than the system.txt had names for.
The mod comes with several system_names.txt file in the DATA folder. You can combine them or use one that has over 255 names.
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Creator of the Star Trek Mod - AST Mod - 78 Ship Sets - Conquest Mod - Atrocities Star Wars Mod - Galaxy Reborn Mod - and Subterfuge Mod.
Atrocities is right. The game only comes with about 100 or so, use the systemnames.txt file that comes with FQM - it has thousands of names - some good and some bad