Why does 'Upload' disappear from modern document library?

Hi guys

I had this exact same issue with some of my users yesterday.  They had some folder and file names beginning with #.  This causes all kinds of errors.  I went in and removed them from all the names, and the upload button appeared again!

Just as an additional heads up, I came across the # in a title issue a few weeks back.  The users were trying to open up a spreadsheet in Excel, and it would appear blank.  It had opened fine in the browser, and there was content in there.  Also, on some of their spreadsheets, they just got 'an error occurred' message and they could only open them in the browser.  I logged a call with Microsoft but they didn't give me the answer.  It was a colleague who had experienced this in a previous role!

I hope this helps someone!