this is a know bug which was fixed with version 27.
The colleagues from development have created a query that selects € and sets it for all users.
Please create a backup before using a SQL on your idoit database.
Dollar = C__CMDB__CURRENCY__DOLLAR
Pound = C__CMDB__CURRENCY__POUND
Schweizer Franken = C__CMDB__CURRENCY__SWISS_FRANC
Select your data database and use
this bug is fixed in i-doit Version 24
Link to Changelog