'a single problem' is correct and usable in written English. You can use it to refer to a specific issue or difficulty that needs to be addressed. For example, "The team encountered a single problem that put the entire project at risk."
"There hasn't been a single problem".
"But we haven't had a single problem".
Water-related risks seldom stem from a single problem.
He added: "Not a single problem was resolved.
I didn't have a single problem within the squad.
"We've never had a single problem with protocol-related issues.
A single problem lies behind all that friction and hostility.
At the lab, students will work together on a single problem.
It will consist of a single problem that also appeared on the individual midterm.
At first, the brain lavishes the scarce resource of attention on a single problem.
"We didn't have a single problem with that," he said gleefully.
Awesome tool! I started using it one year ago and I never had to look for another app
Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia