For some reason in SageMath 8.1 the following code leads to a freeze:
p10 = Posets(10)
p10[18]
I have tried different indices, and everything works fine for i<=17, but starting from 18 I can not access to p10[i] without an immediate freeze. The bug does not depend on system: I tried macOS and Windows 10, both lead to the same result.