Supermicro IPMI KVM: connection failed

What do you know; when I did ipmitool mc reset cold, it did work again.

The only problem is, that I can't do this on the machine I lost the connection to...

For some reason any version of the IPMIView doesn't work with IPMI firmwares >= 3.32, including the last IPMIView one. You have to use the browser launching their Java plugin from a web-interface, and this has to be a specific Java version. This isn't fixed as of 3.43 version (for X10 controller at least), the only difference is that 3.32 version sends RST immidiately after receiving SYN on port tcp/5900, and 3.43 version passes the three-way handshake for tcp/5900 and sends RST 35 seconds afterwards -and that's what Connection failed Java error really means.

Another approach would be to downgrade IPMI firmware to a less recent verion, for example on X10 controller (this is mentioned in the firmware naming scheme - for instance, REDFISH_X10_343.zip) version 3.27 is working with IPMIView. IPMI firmwares for most recent Supermicro motherboards can be found here, you'll notice that Supermicro provides only one IPMI firmware version for a given motherboard, but you can download any firmware version as long as you stick with your BMC controller model - i.e., if your IPMI firmware file is named REDFISH_X10_XXX.zip, you can download and install any X10 version as long as it's for X10. Same rule applies to X11 and other controllers, except that I didn't try it personally, so there still may be some complications.

This way I successfully restored KVM connectivity on a X10DRU-i+ motherboard.

Update: Supermicro TechSupport insists that for firmware versions >= 3.32 a latest IPMIView should be used. Didn't had the chance to check it myself, but just for those who came here in search for a solution - feels like I should mention it.