Thanks for fixing it in v6.6. There are still a bunch of warnings, but the GUI / correction works now.
WARN o.l.l.i.DefaultLanguageIdentifier fastText not configured - language detection performance will be degraded. See https://dev.languagetool.org/http-server#starting-from-command-line for instructions.
WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
WARNING: sun.misc.Unsafe::objectFieldOffset has been called by com.google.common.util.concurrent.AbstractFuture$UnsafeAtomicHelper (file:/usr/share/java/languagetool/libs/guava.jar)
WARNING: Please consider reporting this to the maintainers of class com.google.common.util.concurrent.AbstractFuture$UnsafeAtomicHelper
WARNING: sun.misc.Unsafe::objectFieldOffset will be removed in a future release
WARNING: A restricted method in java.lang.System has been called
WARNING: java.lang.System::load has been called by com.sun.jna.Native in an unnamed module (file:/usr/share/java/languagetool/libs/jna.jar)
WARNING: Use --enable-native-access=ALL-UNNAMED to avoid a warning for callers in this module
WARNING: Restricted methods will be blocked in a future release unless native access is enabled