This is the same error that was seen in earlier versions as FRM-92102. In short, it means that the runtime either crashed or never started. In your case, given that the form is/was running, it means the server (frmweb) crashed. This can occur for a variety of reasons, however is many cases it is the result of not properly generating your application. Try regenerating your X files (FMX, MMX, PLX) using the compile_all=yes option.
If you are generating from the Builder, select the module to be generated in the Object Navigator then press Ctrl+Shift+K, then Save, then Ctrl+T. Be sure to do this for all modules. It is also recommended that you do this in this order PLL first, MMB second, FMB last.
If that doesn't not work, ensure that you are running the latest version, 184.108.40.206