What does the word "于" mean when it's used along with "不容'?
于 in your first sentence means 在。

Therefore, 不容于此 means would not be contained here, although this sentence seems weird without context.

于 is not necessary for 不容，for example, you could say


  这样的行为绝对不容姑息


And it means actions like this will never be allowed in any sense.

于 is neutral, you would also see


  我生于 1997 年


It simply means I was born in 1997 and has not emotional implication.