MySQL: problem when connecting some hosts

I have a problem with connecting mysql on our main db server with other hosts we run.

NOTE: The problem does appear now while we have had to migrate our networks and IP addresses, presumably caused (or evidenced) by this migration.

Migration context: most of our machines have been migrated to (ip.1 on eth0) to (ip.2 on eth0, ip.1 on eth0:0)
This seems to have gone OK, but now I must remove the eth0:0, and some problem do appear. Most presumably some problems of the db machine being aware of its 2 IPs, and allowing connection from the outside only on its old eth0:0
Error message 1:

The mysqli error was: Can't connect to MySQL server on 'xxx.fr' (110).

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Error message 2:

#2003 - Can't connect to MySQL server on 'xxx.fr' (110)

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- I have disabled IPTables filtering and denyhosts
- I have commented the line bind-address      in my.cnf
- etc

All suggestions and ideas are welcome.
How can I find or trace the blocked incoming connexions? nothing appears  in the usual logs..