The idiom "chink in one's armor" refers to an area of vulnerability Wikipedia
Unfortunately, while a 'chink' can be a weakness, in recent times it has become a derogatory statement. This makes the phrase "chink in ones armor", or extended versions ambiguous in meaning.
“Chinks in special ops’ digital and physical armor poses challenges, experts say.”
Additionally, I've seen references to talking about the chinks in a plan or legal defense, where the intent is to compare these to actual armor.
In context it can be use innocuously, such as the US Army talking about actual weaknesses in its special forces armor, or in a more questionable way when referring to an Asian basketballer as a weakness on a team.
This leads to the potential of a double meaning of a statement, so having an alternate single-word would be useful, especially when talking about weakness in other things.
Alternatives such as "Achilles heel" seem to long and are hard to ease into speech, "weakness" has too broad a meaning, a "flaw" seems to imply a failure in design rather than a more passive term, and "spanner in the works" implies sabotage.