Errors during server migration

[testani]

Hi all,
I'm having big problems migrating from Release 8.8.15.GA.3869.UBUNTU14.64 UBUNTU14_64 FOSS edition to 8.8.15.GA.4179.UBUNTU20.64 UBUNTU20_64 FOSS edition, Patch 8.8.15_P43.
I followed the usual procedure which involves moving the opt with rsync, fixing the permissions and reinstalling with the -s option.
All the services start but then I have two problems: the administration interface is a blank page and the webmail gives me a network service error.
Using zmprov via the console gives me ERROR: service.PROXY_ERROR (error while proxying request to target server: Service Unavailable)
com.zimbra.common.service.ServiceException: error while proxying request to target server: Service Unavailable

Can someone help me?