Stock function and cell formatting

Stock function and cell formatting

Hello Forum,

I am trying to fetch some stocks in a sheet. I use a brand new sheet, set an entire column to currency € and fill it with =STOCK("xxxx.xx"). At first it works great, all cells show the stock price correct in EURO. I can save the sheet and access it later without problems. However after a week or so the cells start to show garbage, it first looks like strange dates and when I try to set them back to Euro, they show astronomical numbers. If I delete the cells and re-enter the formulas, it is still not correct.
But if I create an entire new sheet, everything is fine again. The attachment shows in column A the newly created formulas (line 14 and 15 already start to not work...), and in column B what they will look like after a week or so. There are some cells in column B showing the right value, but without an "€", if I ty to set those back to EURO nothing happens at all.

I have no idea what could be wrong...