Adam,
please check the database parameter "_UNICODE" in your SAP DB 7.4.03 version and in your MaxDB version 7.6 and let me know the setting.
This parameter defines if ASCII or Unicode is used for the internal Database Catalog. I was assuming that the database Catalog is automatically migrated to Unicode starting with SAP DB 7.4.03 and therefore that your database is already running with Unicode catalog.
Now I am curious which Catalog version your database is using?
VERY IMPORTANT: do *never* manually change the _UNICODE parameter, because it would not migrate the catalog, which means an ASCII catalog would be incorrectly read as Unicode type...
This parameter is changed implicitly after the database Catalog has been migrated via 'dbmcli -u... -d... db_migratecatalog'.
Up to my knowledge, the db_migratecatalog command can only be executed in versions below MaxDB 7.7, therefore it is important that you first migrate your database catalog.
Older SAP DB and MaxDB versions allow both ASCII or UNICODE catalog, but since MaxDB 7.7 the Unicode version is required and the UseMultiVersionReadSupport error you had posted earlier indicates that a ASCII catalog MaxDB backup was to be recovered into a Unicode catalog version, which is not allowed.
If my assumption is correct and you are indeed still running with the ASCII catalog, I would recommend to test on your MaxDB 7.6 the 'db_migratecatalog' command (do not interrupt, can take a while to complete...) and then try again to proceed further to 7.8.
Please note that MaxDB 7.5 is no different here from version 7.6...
Thorsten
Edit:
Let me add that the 'db_migratecatalog' temporarily disables log writing and therefore you will be required to take a full data backup afterwards...