Why I am getting java.lang.AbstractMethodError errors?

It usually means that you are using an old version of an interface implementation which is missing a new interface method. For example java.sql.Connection interface got a new getSchema method in 1.7. If you have 1.6 JDBC driver and call Connection.getSchema you will get AbstractMethodError.

The simple answer is this: some code is trying to call a method which is declared abstract. Abstract methods have no body and cannot be executed. Since you have provided so little information I can't really elaborate more on how this can happen since the compiler usually catches this problem - as described here, this means the class must have changed at runtime.

From documnentation of AbstractMethodError

Thrown when an application tries to call an abstract method. Normally, this error is caught by the compiler; this error can only occur at run time if the definition of some class has incompatibly changed since the currently executing method was last compiled.

A kind of special case of the above answer.

I had this error, because I was using a spring-boot-starter-parent (e.g. 2.1.0.RELEASE uses spring version: 5.1.2.RELEASE) but I also included a BOM, that also defined some spring dependencies, but in an older version (e.g. 5.0.9.RELEASE).

So one thing to do, is check your dependency tree (in Eclipse e.g. you can use the Dependency Hierarchy) if you are using the same versions.

So one solution could be that you upgrade the spring dependencies in your BOM, another one could be that you exclude them (but depending on the amount, this could be ugly).