Unable to update SQL Server via Access, Reserved error (-7776)
Access (I guess) does not seem too fond of SysStartTime and SysEndTime columns. If I mask them in a view and link the view in Access, both update and insert works. Example:
DROP VIEW dbo.PHYTOCLASS_DIVISION;
CREATE VIEW dbo.PHYTOCLASS_DIVISION as 
select  d.DivisionName
        , c.ClassID
        , c.ClassName
        , c.DivisionID as classDivisionId
        , c.ModifiedBy
        , c.ModifiedDate
        , c.Avslutad
from dBotnia20200929.dbo.PHYTODIVISION d
join dBotnia20200929.dbo.PHYTOCLASS c
        on d.DivisionID = c.DivisionID;

For a trivial join like this, SQL Server figures out how to handle insert/update so instead of triggers are not necessary.
The answer in is-is-possible-to-update-sql-server-2016-temporal-tables-from-access indicates that the problem has to do with the precision of the timestamps in Sys... columns. I have not verified that lowering the precision would help, but it may be worthwhile for someone to read that answer