This is a pretty broad question.  I would start out with the report that you are trying to generate.  
What tables does the report use?  What are you trying to do?

The most common problem I see is that I have used Short Text in an ID field that links to an Auto Number field in another table.

Second, if you are doing math on a field, check that those fields are the same data type.

If this doesn't help you get started we may need more details, including screen shots of where this is happening.  What is the exact message and when does it occur?

Go watch the Type Mismatch FastTip video. That may help explain the error better than I just did.

Hi Scott, thanks for the tip and the quick response. The strange thing is though that the report was working fine until last week and I have not changed anything in the database. I assume that some data in the linked tables could be wrong since many different people work with these. But then again why do all the queries and subreports work?
I will watch the video and hopefully find a solution to this problem there.

Marina if it was working before and just suddenly quit I would suspect the case that Richard points out in the video above where a user put in an invalid response like "N/A".

Are you getting any other detail other than a mismatched date message box?  Does it take you to the code and highlight in yellow where the error is?

Once we figure out where the error is we can figure out how to fix it and also how to prevent it in the future.

I also suspect that someone put an invalid value in one of the linked tables. But why are the queries and everything else work fine and just in the report it gives an error message?

No, it does not take me to the code, just the Message Box Data type mismatch in criteria expression.

I guess I have to check all the approx. 50000 record sets 😒

Thanks for your support.

Hooray, I found the mistake. Someone in fact put two quantities in one field. I found out by just deleting one field of the report after the other and running the report and if the error still appeared I put the field back in and tried the next one. This way I found that it had to be the quantity field, that causes the error.

Now I only have to find out how to prevent people to put more than one quantity in that field. And why the query still runs fine and gives no error message....

That is fantastic news.  Glad that you found it pretty easily.
Here is one thing to check.  On the form where you enter the quantity, make sure the Enter Key behavior is set to Default.
Not New Line in Field.  That could be one place where an error could occur.

See below:

Hi, thanks so much, that is exactly what I was looking for to prevent the error. I have already implemented it and tested it and it works!!

Wonderful!! Don't you love it when the fix is easy?  Glad that helped you out.

Yessss, I love it. I wish it was always that easy.... Thanks again