The issue can be a conflict between a java version and a javaassist version. If you’re using the java 8 on your server you should make sure to use the latest javassist version as well. This blog shows the origin of a conflict being inside the thymeleaf which pulls in an older javassist, the solution being to exclude it from the dependencies
<dependency>
<groupId>org.thymeleaf</groupId>
<artifactId>thymeleaf</artifactId>
<version>${thymeleaf.version}</version>
<exclusions>
<exclusion>
<artifactId>javassist</artifactId>
<groupId>org.javassist</groupId>
</exclusion>
</exclusions>
</dependency>
your case could be different, so you should inspect the dependency tree to find a suitable solution