You can wipe you Spark profile and start with a fresh one. Spark profile is here
C:\Users\username\AppData\Roaming\Spark
If you need your history, copy this folder:
C:\Users\username\AppData\Roaming\Spark\user\username@servername\transcripts
And copy it back after you start with a fresh profile.
Also you can try the latest Spark build http://bamboo.igniterealtime.org/artifact/SPARK-INSTALL4J/shared/build-665/Insta ll4j/spark_2_7_0_665.exe