Java compile error Mendix 6.10.3

Working on an app having run it locally many times without errors.

Suddenly it will not complie anymore given this error message:

Buildfile: C:\Users\worker\Documents\Mendix\EMB2020-main\deployment\build_core.xml

compile:
    [javac] Compiling 204 source files to C:\Users\worker\Documents\Mendix\EMB2020-main\deployment\run\bin
    [javac] C:\Users\worker\Documents\Mendix\EMB2020-main\javasource\communitycommons\Misc.java:656: error: unreported exception COSVisitorException; must be caught or declared to be thrown
    [javac] 		overlay.overlay(new HashMap<Integer, String>()).save(baos);
    [javac] 		                                                    ^
    [javac] Note: C:\Users\worker\Documents\Mendix\EMB2020-main\javasource\com\mendix\core\Core.java uses or overrides a deprecated API.
    [javac] Note: Recompile with -Xlint:deprecation for details.
    [javac] Note: C:\Users\worker\Documents\Mendix\EMB2020-main\javasource\com\mendix\core\Core.java uses unchecked or unsafe operations.
    [javac] Note: Recompile with -Xlint:unchecked for details.
    [javac] 1 error



BUILD FAILED
C:\Users\worker\Documents\Mendix\EMB2020-main\deployment\build_core.xml:27: Compile failed; see the compiler error output for details.

Total time: 2 seconds

Before the last run of this app and this attempt to run it (say about 30 minutes)  I have not made any changes.

Now we are stuck, because this error prevents compilation.

I am not an expert in java, Eclipse or that ilk, but I really appreciate a helpfull explination what may have caused this and how to get rid of it.

TIA.