The sentence structure with the copula 'be' requires a complement.
An adjective, a noun or a noun phrase must follow is .
But there is none of these things in the sentence.
The problem is with the word "often".
It is an adverb (of frequency) and as such it is used for modifying action verbs or adjectives (none of which is the case).
As an adverb it cannot be a complement of the sentence subject "it".
A correct sentence would be: It's quite common to hear such rumors these days.
where common is an adjective, hence fulfilling the minimal requirement for completing the phrase
"It is quite...".
ell.stackexchange.com/users/54542/Ashwin Schumann
If you want to use that sentence structure (which is still passive voice) you must use common not often .
"Often" is an adverb that tells us the frequency of an event in time :
She often comes to this restaurant.
The man doesn't often tell the truth.
That species is not often seen in this climate.
"Common" is an adjective that can refer to frequency in time or spatial distribution:
Alligators are common in Florida.
In parts of Los Angeles, it's not uncommon to run into a famous actor while out shopping.
It's common to hear news stories that paint the President in an unflattering light.
Since you use the pronoun "it" as the subject, you must use an adjective and not an adverb.
On the other hand, if you do change it to active voice, you can use "often" to modify the verb "hear":
We quite often hear such rumors these days.
The same sentence using the passive voice: Such rumors are quite often heard these days.
ell.stackexchange.com/users/42184/Andrew
It is quite often to hear...
ungrammatical We can ashcan dummy "it" and
twist that around like this, using to hear nominally, since non-finite forms of the verb have nominal status ( To err is human ): To hear is quite often.
ungrammatical Often is not a valid predicate adjective.
It isn't an adjective, but a word expressing frequency.
Another way of putting it is that adverbs (like often ) do not license an infinitive complement in the way that adjectives like easy and common do:
It is easy to see...
It is not uncommon to hear...
ell.stackexchange.com/users/10264/Tᴚoɯɐuo
I apologize that I haven't the skill to offer a definitive grammatical analysis, but is the case of common vs often similar to less vs. few ?
People regularly confuse less (the quantity of a single item, "my toast has less butter than yours...") with few (the quantity of multiple items, "I have fewer sticks of butter than you do...").
In the OP's context, I believe common would mean "how frequently everybody does it" (a timeless context) and often would mean "how frequently it happens [to me]" (a context based on time).
To maintain the passive voice, the OP's statment would then have to be, "such rumors are heard quite often these days."
Therefore, to answer the OP's question...
Now, here's where the amature really kicks in.
The use of the word often is wrong because it's trying to modify the predicate to hear but should be modifying the subject rumors because you can't quantify to hear but you can quantify rumors .
ell.stackexchange.com/users/56469/JBH
The problem is that if you use the verb "to be" you need to add a predicative , such as a noun or an adjective, to describe what the subject is.
An example using adjectives is
It's quite often sad/scary/reassuring to hear such rumors these days.
And an example using nouns is:
It's quite often a surprise / a nightmare / a miracle to hear such rumors these days.
If you don't notice the problem with the original sentence, maybe the following trick can help.
In all these sentences, the logical subject is the phrase "to hear such rumors these days".
Try to change the order, putting this phrase at the beginning, thus getting rid of the "it", and at the same time removing other unnecessary parts ("quite" and "these days"): the "core" of the sentence is To hear such rumors is often.
Do you notice, now, that something is missing?
It would be natural to ask: "...
It is often what ?"
So, you need a predicative.
A possible solution, as Ashwin Schumann suggested , is to replace "often" (which is an adverb and can't be a predicative) with "common" (which is an adjective and therefore can):
It's quite common to hear such rumors these days.
An alternative solution, if you don't want to change the word "often", is to use another verb, for example "happens" instead of "is": It happens quite often to hear such rumors these days.
This verb doesn't require a predicative and therefore the sentence is correct.
ell.stackexchange.com/users/30156/Fabio says Reinstate Monica
In  this question , which asks for transforming a sentence from passive voice to active, I left a comment as   It's quite often to hear such rumors these days.   However, @Brillig told me that sentence is not grammatically correct.  I want to know which part of this sentence is at odds with grammar. I am saying that because I asked them if my sentence is unidiomatic or ungrammatical and they said it's ungrammatical.
What is wrong in "It's quite often to hear such rumors these days."?
ell.stackexchange.com
<grammaticality-in-context>
ell.stackexchange.com/questions/136173
ell.stackexchange.com/users/21666/Cardinal
An adjective, a noun or a noun phrase must follow "is". The problem is with the word "often", which  is an adverb (of frequency) and as such it is used for modifying action verbs or adjectives (none of which is the case in this example). A correct sentence would be:  "It's quite  common  to hear such rumors these days", using the adjective "common". A right phrase is this: It's quite common to hear such rumors these days.
The structure of a sentence with the "be" copula needs an addition.
You have to follow an adjective, noun or phrase.
But in the sentence there is no such thing.
The problem is the word "often." It is a frequency adverb and is used as such to modify action verbs or adjectives (none of which is the case). Like an adverb, the phrase "it" cannot be supplemented.
A right phrase is this: It's quite common to hear such rumors these days.