To fix it, if you're in VIC, then in the Navigator click the Goto tab, then click 'Configuration'. Pull down the Actions menu and click 'Reset "My Activities" flag'. If you're not in VIC, then select the "VIC Config" button on your Workspace, pull down the Actions menu and click 'Reset "My Activities" flag'. (more on this below)
VIC prevents multiple instances of this page by using an environment variable. The error you see indicates that the system crashed or was killed (as with killnotes.exe) while the "My Activities" tab was displayed, leaving that flag in the Notes.ini file. VIC won't open it because it thinks it's already open. I'd much rather identify and pull up the existing page if it exists than resort to this stoneage technique, so if someone has a solution I'd be most grateful for the donation of code.
As to being able to bookmark the page… yes, you can. I use it as my home page when I enter Notes. Open the "My Activities" page, then click on some white space on the page so Notes knows you're about to refer to that frame. Then right-click the tab and bookmark it. You can then set the bookmark as your Home, if you like. Keep in mind that if you crash you'll revert to the default. You can then just go to the Workspace, clear the flag, and restart Notes to put it right.
Some entrepeneurial soul might even want to put together a small shell script or EXE that removes "$CratchitWelcomeOpened=1" from the Notes.ini file. Placing that in a batch file that starts up Notes would prevent the problem.