I checked www.soti.net/oem a moment ago, and there is no  service plug-in for RugGear RG740B or RG760   Are you running the device agent for RG760 on your RG740B?  Did you download the plug-in for RG220EX/310/910?   Different RugGear devices seem to run different Android versions and are very different, rather than close derivatives of the same product series.

As you had also tested to confirm that an pcg with another apk file could be deployed successfully,  I suspect that the service plug-in you sent is not built to run on your RF740B.  You can probably test it simply by copying the apk file to the device (via USB, SD-card or whatever means available) and try to tap on the apk file in any file manager to install it.  I guess there will either be a warning message saying that the installation fails, or your installed apps might crash when attempting to run it.

Thank you for your Reply!
The Plugin I used is labeled "RugGear RG730 & RG740", which in fact does not 100% match my "RG740B". So you might be right in that this is not compatible... I would love to test this with the corresponding APK file, but soti supplies .pcg enterpise packages via soti.net/oem and I do not know if or how the actual .apk file can be extracted from that.

For now, i will proceed to the oem request page to request a suitable MDM plugin for my RG740B.

On a sidenote, the error message "file io error on device" is quite misleading here. Something on the lines of "package not compatible" would be more helpfull (if this can be determined anyway).