I am running Spellcheck on a full presentation It will work until the end of the document, at which point it will appropriately ask if I want to start from the beginning, which I do. It pulls up a word from the beginning and when I click replace the entire LibreOffice just crashes and restarts. I have had to recover the same document twelve times while proof reading it. Eventually I hand proof-read it, because it's college related.
On which LO version are you? Last stable one is 7.6.7 or brand new 24.2.4. Also you can try https://wiki.documentfoundation.org/QA/FirstSteps