Excel freezes till I save another worksheet

[sward-uk]

Strange one; I can be working on a spreadsheet (Excel 365 on desktop) and it will no longer let me click into a cell or enter data etc. The only two ways to 'unfreeze' are to move to another open spreadsheet, save that one and then revert back to the one that was not accepting any actions, or close the spreadsheet and reopen.

• leaving it for a period of time does not help
• saving the inactive sheet does not help - it needs to be another open sheet
• It can happen to any open spreadsheet and I have not managed to find any particular sheet that is open when this happens
• It might happen once a day or several times in 5 minutes

Any thoughts oh wise ones?