I just checked something in the new and updated rules for descent and I do feel like this is a pretty major change.

It concerns the Golden rules, specifically the first one, I shall paste the old and new versions for clarity:

Many quest rules listed in the Quest Guide come
in direct conflict with the rules found in this
rulebook. The quest rules listed in the Quest Guide
take precedence over the rules in this rulebook.
Furthermore, some cards and abilities will come in
direct conflict with rules found in this rulebook and
the Quest Guide. Cards and abilities take precedence
over both the Quest Guide and this rulebook.

And now the new one;:

Some cards and abilities come in direct conflict with

the rules found in this rulebook. Cards and abilities

take precedence over the rules in this rulebook.

Furthermore, many quest rules listed in the Quest

Guides come in direct conflict with cards, abilities,

and the rules found in this rulebook. The quest rules

listed in the Quest Guides take precedence over cards,

abilities, and the rulebook.

...so for example, now you could not use Dash on Scarn in "Spread your Wings", since he may only take one move action, but you still could play Blinding speed on him.

...I think this is a very stupid idea, and likely it will not be the only case. It is a slight change, but it is actually rather far reaching, I would have to check how many scenarios might be drastically changed just by this ruling.

Has anyone asked about this change already maybe?

The change is definitely intentional (not a misprint.) There are a number of instances where the rules in the quest guide conflict with cards but there had already been rulings causing the quest guide to take precedence anyway. Most of these were situations where the quest guide included a phrase like ,"by any means." The errata change just codifies that. There are scenarios (like you pointed out in "Spread Your Wings") which are in fact different now.

I wish they would just use clear phrasing that avoids such technicalities...

Like "only one of this figures two regular actions can be a move action".

...well, one lesson for better rule design at least, but I would like to know when the previous rules where that troublesome.

Yeah though it kinda makes some OL cards in some quests useless. I guess OLs should choose the card in their decks wisely.