Net names that contain "non-English" characters such as accents, umlauts, or cedillas are not accepted in ECS.
When these characters are used, the schematic editor reports that they are illegal. You can work around this problem by using names that do not include these special characters.
In some cases, the schematic editor has allowed schematics using these special characters to be saved; later, however, the schematics cannot be opened. If this is the case, work around the problem by opening the schematic in a text editor and modifying the names to remove the special characters. After you save the schematic, you will be able to open it in the schematic editor.