Since you can edit categories, projects, and tasks on multiple devices, it could happen that Marvin enters into a weird state like a category being its own grandparent. Usually this presents in Marvin as categories/projects/tasks appearing in Inbox when you didn't expect it.
First login at pre. Then follow this guide to open the console. Then type
FIX_CYCLES() and press enter.
You will either get a message saying "No cycles/orphans detected", in which case everything is fine, or a message like "Call FIX_CYCLES(true) to move printed items to inbox". In this case, type
FIX_CYCLES(true) and press enter to have Marvin fix the issues. After the issues are fixed, reload the page, and then check your Inbox and move the fixed tasks/categories/projects to where they belong.