I have had an acer palmatum for about two years. Last year the plant dried up a bit but the problem was solved. In the winter I moved the plant to a larger pot with a drainage hole. In the last few days, some of the leaves on top and bottom started to dry out (are Brown and curled) and some fell off, being much worse than what happened last year. I have been watering regularly, as the soil seems a bit dry. In the morning, the acer is totally in the shade and catches wind, it doesn't start getting direct sunlight until 1pm. Where I live, in a coastal area very close to the sea, it has been moderately warm (temperatures between 22°C and 26°C). I am worried, what should I do?
This is a common problem with Japanese maples. It's called leaf scorch. The reason is too hot sun and not enough water during hot weather. And hot can be above 20C already.
The best thing to do is to find the right spot for your maple. The best spot would be some direct morning sun, and shade for the rest of the day. As I understand, yours gets afternoon sun now, but this is too hot during summertimes. You could try to compensate this with a lot of watering during hot days (but no guarantee that you can keep up with its need for water). So best would be to keep it in the shade during summer (or give it only morning sunlight if possible).
Agree with the other answer, but I'd add its better if it's not in a windy spot too. The damage present definitely looks like a combination of too much sun exposure, especially sun in the early to late afternoon, and possibly insufficient water, but wind can also damage the leaves.
Not an expert on these maples, but generally agree with water issue. However, it looks like perhaps it was over-fertilized? If you have added regular fertilizer without good drainage, all this can accumulate and reach toxic levels. If you fertilized, I would leach it out with clean water (like Reverse Osmosis water) and re-pot in soil containing high organics, like peat moss. Peat is slightly acidic and will help flush out excessive dissolved solids. If your water contains excessive lime, your maple might not like high pH. Higher organic matter content will also help retain more soil moisture. Watering with rain water might also help. If you like to fertilize, I would try using a watered down solution of commercial composted manure. This will look light brown or tea-colored. It adds a lot of organics, good for retaining soil nutrients, and give the plant enough nitrogen to produce growth and chlorophyl. Good luck!