What is this Run-time error?

My program runs for around 50 minutes communicating in a loop over a serial line to a UPS system gathering data. All of a sudden after 232 loops (not always 232), a message pops up saying

Run-time error '8021':
Application-defined or object-defined error

The program has 17,268 code lines, 466 procedures and 1,623 controls. It doesn't always happen.

Any ideas?

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