This related question asks if you could transcribe a ritual spell to the warlock book via the Book of Ancient Secrets invocation regardless of spell lists, and the answers were generally "yes, but you can't tell what the spell is".
This other one asks if you had the Eyes of the Rune Keeper invocation, "could you cast a scroll of a spell outside of your spell list?", and the answer, using a quote from Crawford, was that you cannot cast the spell since it was out of your spell list (but it did not state if you could actually read the scroll).
By this point, you can probably guess where I'm getting at.
The description of the Warlock eldritch invocation, Eyes of the Rune Keeper:
You can read all writing. You can comprehend any written word or symbol, should it hold any linguistic meaning.
The rules on scrolls outside of your spell list:
If the spell is on your class’s spell list, you can read the scroll and cast its spell without providing any material Components. Otherwise, the scroll is unintelligible.
Can I understand magical literature that is normally outside of my class list if I have the Eyes of the Rune Keeper?
What would I be able to identify from a written spell with Eyes of the Rune Keeper?
Note: I am not asking if I can cast spells outside of my class with the invocation; I am asking if I can understand the nature of the scroll/spell, even if I can't cast from it.
Related: Can Warlock's Eyes of the Rune Keeper decipher written code?
Best Answer
No. Eyes of the Rune Keeper Does Not Read Spells.
tl;dr "Generally, you would not know anything about a scroll, even if you can read every language. Scrolls don't use a language." – Daniel Zastoupil
In short, the words (if any) could be understood in a literal sense, but that doesn't get the meaning nor substance of the spell. Spells, such as those on scrolls, don't have linguistic meaning. They are a cipher.
Ciphers are (generally) not pronounceable.
An example simple cipher using rot-13 of "i am a cipher" has the cipher text:
There is no meaningful pronunciation once the letters are rotated. Other ciphers such as digraph substitution are even less intelligible.
No Linguistic Meaning.
In the case a cipher is chosen such that the cipher text is pronounceable, they wouldn't necessarily be words with a linguistic meaning. For example, while pronounceable, the following cipher text is gibberish:
Pronounceable Codes are Cryptolects
While the warlock would be able to technically read and pronounce the words, they would be unintelligible.
The ability does not now endow the possessor with the ability to understand codes. Even if the spell was not strictly a cipher, but a cryptolect analogous to thieve's cant or irish traveller's language, it would still be unintelligible.
As Crawford says in this tweet:
Reading & Casting Clarification by JC
Jeremy Crawford answered a related question about casting in this thread.