CalendarBudget supports multiple currencies which are set both on Accounts and Categories. If there is a mismatch, (ie, the currency symbol on your categories is “$”, but your account is changed to be “€”, when creating/editing an entry in that account, categories with currency “$” will not show up since you cannot mix $ and €.
To fix this, make sure your account currency (Setup -> Account Setup) and your categories (Setup -> Category Setup) match where you expect them to.