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funny this problem was prolongued to version 14.
If possible cancel automatic calculation and try to copy and paste to a new sheet
Check out if the new file was really stored in the File manager (an if it works - may be on a different PC) and afterwards kill the Mathcad Process if it still delivers the errors.- at least for me that worked in V14 on W7...
Try saving it as a .mcd file, rather than .xmcd or .xmcdz.
Thanks for the tip - ist also helped to save as compressed version in contrast to the xmcd format.