- That is an actually sufficient account.
- That is actually a sufficient account.
What is the difference in the meaning of 1 and 2?
Are both 1 and 2 correct gramatically?
If both mean the same, which one is more idiomatic?
Sentence #2 is grammatical and idiomatic.
Sentence #1 doesn't sound quite right. I don't think it's really because of the grammar, since an adverb would be appropriate modifying "is" (#1) or the adjective "sufficient" (#2).
The problem is with the meaning of the combination "actually sufficient". Unless you are using some specialized vocabulary where "actually sufficient" has a commonly accepted meaning, I don't think "actually sufficient" makes sense in English. I'm sure people will correct me if I'm wrong, but I think the adjective "sufficient" is a good descriptor by itself, and "actually" doesn't add anything significant. So I would say sentence #1 wasn't idiomatic.
The difference between the two sentences is simply what the word "actually" is modifying.
In the first first sentence, the modified word is "sufficient", whereas in the second, "is" is modified. Neither is grammatically incorrect, and the only change is which, the action "to be" or the state "sufficient", is being described as actual, and as such which is being highlighted or stressed in the sentence.
Also, noting that the highlighted words in both sentences are "a" and "an", the difference between the two doesn't change anything in the sentence, or the meaning of the sentence. Simply, "a" is to be used before consonants, and "an" before vowels.