Osmo has suddenly stopped running.

There might be something in the config it doesn't like. You could try renaming the config dir. It might be ~/.osmo or ~/.config/osmo or somewhere else.

I have not used Osmo, but what you describe appears to be a bug or very bad practice from the developers.

In C and C++ assertions are a feature intended to be used for internal consistency checks (those that depend only in the program and not in its environment). Sometimes they are used for external consistency checks, like checking that a required file exists. This is incorrect. It is also very bad practice because when the assertion fails it terminates the program suddenly and gives no information for troubleshooting which is exactly what is happening here.

If you changed something relevant, as a workaround try undoing it and try to run Osmo again. You can also try compiling and installing the latest version of gtkspell manually and instructing Osmo to use it. Maybe the bug has already been fixed in the latest version; otherwise, report the bug to the developers of Osmo.