Sometimes, when you do a search, Excel Online loops in a "scroll up, scroll down" loop - that is, it jumps between displaying one set of rows and another, and does not allow you to do anything. You have to shut down the spreadsheet and start again to get out of the loop. This happens in particular when the worksheet is filtered. I don't know if it is influenced by whether another user is working on the same spreadsheet.
Excel online is corrupting my excel workbooks. When I open my excel file several times, saved to onedrive, it clears all the formatting of each and every cell of all the worksheets and locks all of the cells in the workbook.