The sentence is: This weaknesses negated his otherwise progressive attitude towards the staff.
I feel hard to understand the meaning of "otherwise progressive attitude". How to transfer this sentence into a simple one?
Otherwise is an adverb meaning In other respects; apart from that (Oxforddictionaries.com)
an otherwise totally black cat with a single white whisker
Your sentence means
His attitude towards the staff was positive in the whole, except for this weakness.