Which word signals a problem?
actually
process
difficult
inexplicable

Well, that depends on a context, but i find that the last two have a potential to signal a problem, and the second  last (difficult) more than the one after it:

1. He was overcome by inexplicable fear (there is a "problem" in the sentence, but can be attributed to other words).
His homework was difficult (this is a problem, so definitely a safe option)

He takes his shirt off then high fives the mascot but eventually will get a red card