I tested the plugin with on my 64bit Windows 10 machine with Sigil 1.4.3 and the latest LanguageTool release and wasn't able to reproduce your problem.
Download
the latest LanguageTool snapshot, unpack it and update the
ltPath setting in LanguageTool.json. Also double-check that you have installed the latest plugin version.
If that doesn't fix the problem, check whether you have accidentally installed a 32bit Java version.
Open a command prompt, paste the following string into it and hit Enter:
Code:
java -XshowSettings:properties
This will output several lines of text. Scroll back until you see an entry that starts with
sun.arch.data.model. If it doesn't look like the following entry, you have a 32bit Java interpreter:
Code:
sun.arch.data.model = 64