output.csv ist just a text file (comma separated values).
If you are using linux, “less output.csv”. If you are using windows, point your favourite editor at the file. If you are using a Mac, there is time to buy something else.
At the top, you should have comment lines, introduced with a hash. That look like,
# stan_version_major = 2
# stan_version_minor = 14
# stan_version_patch = 0
# model = ...
You should then see a non comment line like
and eventually your values. At the end are some more comments with run-time information.
Check if the first few lines look OK.
Have you moved the file between operating systems ? Depending on what kind of readline()/getline(), … is used, different newline terminators can confuse some readers.