https://github.com/onboard-osk/onboard
Is this the sameproject?
If so, I would post an issue on their github, usually the maintainers will get it fixed. If it's not an issue with Onboard, I would then post on the Linux Mint forums, maybe it's like a permission issue or something.
There's also linuxquestions.org.
*There's a somewhat similar issue (https://github.com/onboard-osk/onboard/issues/41), but that has to do with context and mouse pointers and such.