Font error

I have several spreadsheets linked together (8 of them),
which also include charts.

I keep getting a new error in them, which says "No more
new fonts may be applied in this workbook".

I did not add anything that I am aware of, which required
odd fonts. I primarily use one font; Arial. When I
looked this error up on the support knowledgebase, it
referred me to some chart issues, but I have done nothing
with any charts.

Any ideas on what may be wrong? This set of spreadsheets
has worked for about 4 years, but in the past year, we
have added more tabs to it and more formulas.