You say you're trying to migrate a vdi, where are you trying to migrate it?
To a SR attached to your current host or to another host/pool?
If you're trying to migrate it to another host/pool this is something that i've seen as well when dom0 is not assigned enough RAM, we usually gave them 16Gg which seemed to resolved these kind of issues back when we were running 8.2.0.
@acebmxer
Just update your config as bellow
[http.mounts]
164 # Uncomment to setup a default version.
165 # Otherwise, XO5 will be the default for stable channel and XO6 for latest and source
166 # '/' = '../xo-web/dist/'
167
168 '/v5' = '../xo-web/dist/'
169 '/' = '../../@xen-orchestra/web/dist/'
170
171 [http.proxies]
172 # [port] is used to reuse the same port declared in [http.listen.0]
173 '/v5/api' = '[protocol]//localhost:[port]/api'
174 '/v5/api/updater' = 'ws://localhost:9001'
175 '/v5/rest' = 'http://localhost:[port]/rest'
