Perception of problems when informed of an issue?

I'd like to suggest that you're attempting to delve too far into psychology for what you truly understand just from your own description. I can offer an alternative explanation for observing this behavior that contradicts any of the ones you deemed related:

Users reported performance problems when told we were running on backups because performance was the only thing they weren't sure was working correctly. It was easier for users to report this issue than to test for it accurately.

Rather than pass judgements about why they did this consider a term and wording that sticks to what you know:

Informing our users that we were temporarily running on disaster recovery servers biased them to report performance issues.

This way you've only reported what you've observed.

bias

bi·as

ˈbīəs

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noun

1. prejudice in favor of or against one thing, person, or group compared with another, usually in a way considered to be unfair.

"there was evidence of bias against foreign applicants"

synonyms: prejudice, partiality, partisanship, favoritism, unfairness, one-sidedness; bigotry, intolerance, discrimination, leaning, tendency, inclination, predilection, casteism

"he accused the media of bias"

2. in some sports, such as lawn bowling, the irregular shape given to a ball.

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verb

1. cause to feel or show inclination or prejudice for or against someone or something.

"readers said the paper was biased toward the conservatives"

synonyms: prejudice, influence, color, sway, weight, predispose; distort, skew, slant

"this may have biased the result"

prejudiced, partial, partisan, one-sided, blinkered;

bigoted, intolerant, discriminatory;

distorted, warped, twisted, skewed

"a biased view of the situation"

2. give a bias to.

"bias the ball"

google: bias

Selective Perception seemed the most appropriate; though the more general term Cognitive Bias would also be appropriate.

Selective Perception: The tendency for expectations to affect perception.

Full list of Cognitive Biases can be found here: https://en.m.wikipedia.org/wiki/List_of_cognitive_biases

It may be that your users believe in the

domino theory

dom′ino the`ory

n.

2. a theory that a particular event will precipitate similar ones elsewhere. Also called domino effect, domino reaction.

thefreedictionary

Because one thing went wrong they expect other things to go wrong. Traditionally this term comes from the realm of politics rather than psychology.

While I agree that @JohnLBevan's suggestion of Selective Perception is most fitting, there's also "hyper-awareness" as a related concept.

Schultz described it best: