This is a regression in the randomization of the va setting.
UML will boot on debian 4.19 kernel host with kernel.randomize_va_space = 2 UML will not boot debian 5.2 kernel host with kernel.randomize_va_space = 2 UML will boot on 5.2 once kernel.randomize_va_space is set to 0 on the host.So something has changed in how randomize is implemented between 4.19 and 5.2.
-- Anton R. Ivanov https://www.kot-begemot.co.uk/