Europeans of Lemmy, what places in Europe should foreigners avoid at all cost?

Not sure what answers you looking for, if you want to have a carefree vacation just read some reviews of specific countries and regions.

If you are backpacking or planning a multi-country trip I would check the crime rates of the places, there are many websites with the statistics available, like THIS

As a general rule of thumb popular places and big cities will have the highest crime rates, while smaller cities and countryside the lowest.

Also I would avoid solo trips and backpacking in general in the rural parts of less-developed countries, like Romania, etc.

Pretty much thats it. Europe as a whole is probably one of the safest travel destinations in the earth with some planning and common sense.