As for me it seems like corruption. You can try to use a built-in recovery. I sugest you to do the following
1. Locate the .ost file.
I. From the Tools menu, click Account Settings.
II. From the "Account Settings" window, click the Data Files tab.
III. Click the data file labeled as "Default."
IV. Click Open Folder....
2. Rename the .ost file by right clicking on the file and selecting Rename.
- Suggest to rename the file to ost.old.
3. Press Enter on your keyboard.
4. Open Outlook. Outlook should start to configure a new data file.
- This process can take anywhere from 10-60 minutes depending on your mailbox size.
When your mailbox has been rebuilt you'll see the message, "All folder are up to date" in the lower right hand corner.
If the above mesures by some reasons had no effect. I suggest to use this program Recovery Toolbox for Outlook Express. You can find it here https://www.outlookrepairkit.com/