Finally it works also with media converters! I elaborate on it, and I found that the MS120 wasn't connected to MS220 directly, but it was connected to the HP instead. So: MX --> MS220 -- fiber --> HP -- media converter --> MS120. and the HP had incorrect VLAN configured ports. For instance, HP uses another terminology: a Trunk port that has to transmit packets of some VLAN must be Trunk tagged, mine was Untagged! Thanks to all for the support!
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