"Read-error" repeatedly crashes my sheets

I have a couple of fairly large sheets in LO-calc that I am building and I am having a real hard time because they keep crashing and losing work. The complete error message that I get is:

Format error discovered in the file in sub-document content.xml at 2,277347(row,col)”

I am running Libre Office Version: on Windows 7. I speak american english
A few days ago I began going in into versions and saving versions anytime I made a significant structural change but that turned out to be useless any time I actually need it.
I suspect that it may be the result of the fact that I have been using a lot of comments, sometimes I have been pasting in fairly large groups of html text from the web that I then gradually slice and dice into the appropriate columns. Could this be the problem?

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