Hi Malin,

This is Create script for ActivityLog table on Episerver CMS 10:

CREATE TABLE [dbo].[tblActivityLog](
	[pkID] [bigint] IDENTITY(1,1) NOT NULL,
	[LogData] [nvarchar](max) NULL,
	[ChangeDate] [datetime] NOT NULL,
	[Type] [nvarchar](50) NOT NULL,
	[Action] [int] NOT NULL,
	[ChangedBy] [nvarchar](255) NOT NULL,
	[RelatedItem] [nvarchar](255) NULL,
	[Deleted] [bit] NOT NULL,
 CONSTRAINT [PK_tblActivityLog] PRIMARY KEY CLUSTERED 
(
	[pkID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

ALTER TABLE [dbo].[tblActivityLog] ADD  CONSTRAINT [DF_tblActivityLog_Action]  DEFAULT ((0)) FOR [Action]

ALTER TABLE [dbo].[tblActivityLog] ADD  DEFAULT ((0)) FOR [Deleted]

You can manually add missing columns to ActivityLog table to avoid losing data. 

If this still doesn't work, please send me episerver log so I can investigate deeper solution.