How is this possible?
An Inventory Item and a Sales Invoicein a single history entry does not make any sense. History is not supposed to work this way.
What have you done to get this exact result? It could be related to the reason why you cannot Undo this particular entry.
I use the usual software tools. And I donāt understand how it happened either.
Restore your backup that you have and look at that
You must have used a batch update as that is when more entries show in one history screen. If you open one of a batch it will show all related that were altered as well. The cool thing is that when you do undo (only when starting from latest history point) it will undo all related ones also.
I canāt understand how it happened. And how can it be avoided in the future?
Now I am manually adjusting (undo) hundreds of transactions since 2017. Then I have to check monthly turnovers and balances. Crazy job.
You would not have any such stress if you make backups as @Patch explained earlier. Also the error you face is one made by you or someone in your business, it is so called user error. You do not have go one by one if the error is due to a batch update procedure. As I mentioned before start undoing from the most recent, at some point you will find the culprit as the screen will show many changes. Then when you undo that, all the other ones in history belonging to it will also be undone.
When you go to the History screen, do you see a button at the bottom-right of the screen that says Fix History ?
No have not a button fix history
If you have not done anything to it, then it looks like some kind of data corruption.
The only thing to do is to email a backup to lubos@manager.io
That may be an option if he has a backup which has not been extensively modified by the user after the anomaly was discovered. And a copy of the data file which behaves as expected.