There are two main problems with the original sentence.
(1) The subject of this sentence (financial problems) is plural and does not agree with the verb (ranks), which is singular. Do not be thrown off by the appositive between the subject and verb that is designed to confuse you.
(2) The phrase rank as one of the high causes is not idiomatically correct. Instead, itemsrank as one of the leading causes.
A. The subject (which is plural) and verb (which is singular) do not agree; ranks as one of the high causes is awkward and unidiomatic
B. This sentence correctly uses a plural verb with its plural subject; the concise and idiomatically correct phrase rank as one of the leading causes is used
C. The phrase rank as one of the most high causes is awkward, unidiomatic, and redundant (the words most and high convey the same idea)
D. The subject (which is plural) and verb (which is singular) do not agree
E. The phrase rank as one of the most leading causes is awkward, unidiomatic, and redundant (the words most and leading convey the same idea)