try upgrading gradle

build.gradle

@@ -13,7 +13,7 @@ plugins {
  id 'me.champeau.gradle.jmh' version '0.5.2'
  id 'com.github.ben-manes.versions' version '0.36.0'
  id 'org.javamodularity.moduleplugin' version '1.7.0'
- id 'org.openjfx.javafxplugin' version '0.0.9'
+ // id 'org.openjfx.javafxplugin' version '0.0.9'
  id 'org.beryx.jlink' version '2.23.1'
  // nicer test outputs during running and completion
  // Homepage: https://github.com/radarsh/gradle-test-logger-plugin
@@ -38,11 +38,13 @@ version = project.findProperty('projVersion') ?: '100.0.0'
 java {
  sourceCompatibility = JavaVersion.VERSION_14
  targetCompatibility = JavaVersion.VERSION_14
+
 }
 
 application {
  mainClassName = "org.jabref.gui.JabRefLauncher"
 }
+modularity.disableEffectiveArgumentsAdjustment()
 
 // TODO: Ugly workaround to temporarily ignore build errors to dependencies of latex2unicode
 // These should be removed, as well as the files in the lib folder, as soon as they have valid module names
@@ -87,10 +89,6 @@ configurations {
  }
 }
 
-javafx {
- version = "15"
- modules = [ 'javafx.controls', 'javafx.fxml', 'javafx.web', 'javafx.swing' ]
-}
 
 dependencies {
  // Include all jar-files in the 'lib' folder as dependencies
@@ -145,6 +143,12 @@ dependencies {
  implementation group: 'jakarta.annotation', name: 'jakarta.annotation-api', version: '1.3.5'
 
  // JavaFX stuff
+
+ // https://mvnrepository.com/artifact/org.openjfx/javafx-controls
+ implementation group: 'org.openjfx', name: 'javafx-controls', version: '15.0.1'
+ implementation group: 'org.openjfx', name: 'javafx-fxml', version: '15.0.1'
+ implementation group: 'org.openjfx', name: 'javafx-web', version: '15.0.1'
+
  implementation 'de.jensd:fontawesomefx-commons:11.0'
  implementation 'de.jensd:fontawesomefx-materialdesignfont:1.7.22-11'
  implementation 'de.saxsys:mvvmfx-validation:1.9.0-SNAPSHOT'

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\:https://services.gradle.org/distributions/gradle-6.5-bin.zip
+distributionUrl=https\:https://services.gradle.org/distributions/gradle-6.7.1-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists

gradlew

@@ -130,7 +130,7 @@ fi
 if [ "$cygwin" = "true" -o "$msys" = "true" ] ; then
  APP_HOME=`cygpath --path --mixed "$APP_HOME"`
  CLASSPATH=`cygpath --path --mixed "$CLASSPATH"`
-
+ 
  JAVACMD=`cygpath --unix "$JAVACMD"`
 
  # We build the pattern for arguments to be converted via cygpath

gradlew.bat

@@ -40,7 +40,7 @@ if defined JAVA_HOME goto findJavaFromJavaHome
 
 set JAVA_EXE=java.exe
 %JAVA_EXE% -version >NUL 2>&1
-if "%ERRORLEVEL%" == "0" goto execute
+if "%ERRORLEVEL%" == "0" goto init
 
 echo.
 echo ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
@@ -54,7 +54,7 @@ goto fail
 set JAVA_HOME=%JAVA_HOME:"=%
 set JAVA_EXE=%JAVA_HOME%/bin/java.exe
 
-if exist "%JAVA_EXE%" goto execute
+if exist "%JAVA_EXE%" goto init
 
 echo.
 echo ERROR: JAVA_HOME is set to an invalid directory: %JAVA_HOME%
@@ -64,14 +64,29 @@ echo location of your Java installation.
 
 goto fail
 
+:init
+@rem Get command-line arguments, handling Windows variants
+
+if not "%OS%" == "Windows_NT" goto win9xME_args
+
+:win9xME_args
+@rem Slurp the command line arguments.
+set CMD_LINE_ARGS=
+set _SKIP=2
+
+:win9xME_args_slurp
+if "x%~1" == "x" goto execute
+
+set CMD_LINE_ARGS=%*
+
 :execute
 @rem Setup the command line
 
 set CLASSPATH=%APP_HOME%\gradle\wrapper\gradle-wrapper.jar
 
 
 @rem Execute Gradle
-"%JAVA_EXE%" %DEFAULT_JVM_OPTS% %JAVA_OPTS% %GRADLE_OPTS% "-Dorg.gradle.appname=%APP_BASE_NAME%" -classpath "%CLASSPATH%" org.gradle.wrapper.GradleWrapperMain %*
+"%JAVA_EXE%" %DEFAULT_JVM_OPTS% %JAVA_OPTS% %GRADLE_OPTS% "-Dorg.gradle.appname=%APP_BASE_NAME%" -classpath "%CLASSPATH%" org.gradle.wrapper.GradleWrapperMain %CMD_LINE_ARGS%
 
 :end
 @rem End local scope for the variables with windows NT shell