Custom extensions are extensions that you create, such as to customize the layout of an invoice, that do not link to the banana.ch site but are saved in a local folder.
To use this type of extension you must have the version of Banana Accounting + Advanced plan, or use the application in Demonstration mode, for example limiting accounting entries to 70 lines.
Special cases
If the extension is a Banana.ch extension and the error appears after renaming the user of the operating system or transferring Banana Accounting+ to another computer, try uninstalling and reinstalling the extension indicated in the error message.