I've made as suggested.
1 - Saturday morning started Unraid with only array up and docker down.
2 - At 12:00 AM I've started Docker DuckDns, Swag and MariaDB. Nextcloud off. No problems for all the next night.
3 - Sunday morning at 8:00 AM I've seen that network for MariaDB was set to Bridge instead of the other attached docker (Nextcloud and Swag have network setting with a custom network "proxy net"). Set network for MariaDB same as Nextcloud and Swag.
4 Now is Monday 10 18:54 PM and all works fine. No more problems.
I think I've found the problem.
Thank you.