Sometimes Excel just stops processing certain formulas on a worksheet and
considers the as plain text (regardless of the fact that the cell contents
starts with an equation sign "="). For example formula "=1+2" doesn't show
anymore as "3" but as "=1+2", as if it's only a string of text .But if I type
the same formula in the cell next to it, it shows the correct result "3".

Also, when I try to evaluate the formula, excel tells me that "the cell
currently being evaluated contains a constant" -- so its not about wether my
formulas are "visible" (tools-options-view-formulas) or not. The number of my
formulas asre always under the maximum 1024 characters.