I'd like to know whether it is grammatical in contemporary English to modify an uncountable noun with ordinal numbers.
Is it right to say, "first disobedience" or "second importance"?
My proposed answer: A solid maybe - though the meaning is usually more specific than the use without an ordinal number. It is not very common and is typically a shortened way to refer to an already discretized version of the word (eg. dropping "pieces")
This site has a list of uncountable nouns. What follows are examples of what it lists as "uncountable words" being used with ordinal numbers, followed by an alternate phrasing that introduces a countable noun:
At the first time increment the probability of failure of the first equipment is calculated.
... first piece of equipment ...
This marks the first evidence of Denisovans found outside Denisova Cave in Siberia since the mysterious ancient human group was discovered in 2010.
... the first piece of evidence ...
On today's edition of Forza Monthly, the studio's own Brian Ekberg confirmed that the first news of the project will be coming on May 7 in the next edition of the show.
... the first piece of news ...
It was a beautiful winter's day, bright and crisp, the [first sunshine] London had seen in weeks.
... the first day/moment of sunshine ...
Of second importance are the three anti-PM/Scl-positive DM patients.
The second most important items ... [could not think of a better rephrasing for this]
As you can see there are cases where an ordinal number can be used with an uncountable noun. The most common usage (and easiest to find) are ones using "the first [uncountable]" format, that refer to something that is/was "the first in time". This usage is context- and word-specific as to whether it is "grammatical" or "idiomatic".
As with most English grammar rules, there are exceptions to the rule that ordinal numbers are not used with uncountable nouns.