SOLVED: Interfaces vanish after /interface ethernet set

I have a RB750GL and have the problem on both ROS 5.14 and 5.15.

I am trying to change from within a script the mtu of my lan interfaces.

This is the code I am running from within the “Run After Reset” script of system reset configuration:

# wait for interfaces
:while ([/interface ethernet find] = "") do={ :delay 1s; };

/interface ethernet
set 0 arp=proxy-arp auto-negotiation=yes bandwidth=unlimited/unlimited disabled=\
    no full-duplex=yes l2mtu=1520 mac-address=00:0C:42:AA:7B:CE master-port=\
    none mtu=1492 name=ether1 speed=100Mbps
set 1 arp=enabled auto-negotiation=yes bandwidth=unlimited/unlimited disabled=\
    no full-duplex=yes l2mtu=1520 mac-address=00:0C:42:AA:7B:CF master-port=\
    none mtu=1492 name=ether2 speed=100Mbps
set 2 arp=enabled auto-negotiation=yes bandwidth=unlimited/unlimited disabled=\
    no full-duplex=yes l2mtu=1520 mac-address=00:0C:42:AA:7B:D0 master-port=\
    none mtu=1492 name=ether3 speed=100Mbps
set 3 arp=enabled auto-negotiation=yes bandwidth=unlimited/unlimited disabled=\
    no full-duplex=yes l2mtu=1520 mac-address=00:0C:42:AA:7B:D1 master-port=\
    none mtu=1492 name=ether4 speed=100Mbps
set 4 arp=enabled auto-negotiation=yes bandwidth=unlimited/unlimited disabled=\
    no full-duplex=yes l2mtu=1520 mac-address=00:0C:42:AA:7B:D2 master-port=\
    none mtu=1492 name=ether5 speed=100Mbps

After doing the reset I can connect using winbox however winbox shows empty windows: there are no interfaces at all, I cant get a terminal, there is nothing. Most of the time the winbox connection breaks after a minute or so.

Am I doing something wrong or is this a bug that I should report?

I have solved my problem.

The problem is that you cannot use winbox to access the RB on any interface with an mtu of less than 1500.

The winbox documentation ( http://wiki.mikrotik.com/wiki/Manual:Winbox) does not mention this.

There is a note on the first time setup page http://wiki.mikrotik.com/wiki/Manual:First_time_startup where it says

This method works with any device that runs RouterOS. Your PC needs to have MTU 1500