What kind of problems, in what kind of circumstances ?

Nausea, vomiting, weakness, frequent urination, kidney stones. It's really hard to get too much Vitamin D from food (doesn't have enough) and sun exposure (the body usually regulates itself well enough) so it's primarily supplements that cause Vitamin D toxicity unless you have other medical issues going on. It's definitely a supplement that's better taken under guidance of a physician who can order labs to monitor serum levels.

EDIT: It's worth noting that it takes a very high and frequent dose (60,000 IU daily, for instance, can cause it which is about 100x the recommended daily allowance of 600 IU).

Thanks. Couple friends got diagnosed with vitamin D deficiency after going through Covid, both got 1-2k i.u, a day. But 60K? Good god. That really seems like overdoing it !!!

NP. Yeah, I was recently tested and found very deficient and even my doctor didn't prescribe anything close to that level. I'm at ~14k/day (50k twice a week) for 12 weeks. Talking with my colleagues, it seems most are on a prescribed Vitamin D supplement (or an encouraged supplement), but no one else was even close to my level. One has a 10,000 IU daily dose for 8 weeks, the rest have 1k-2k (like your friends) indefinite daily dose or just for the winter to carry them through.