This line of StandardRegimenTagHandlerSupport1_10 references the class RegimenSuggestion:
attributeDescriptors.add(new AttributeDescriptor("regimenCodes", RegimenSuggestion.class));
And RegimenSuggestion was removed from Core 2.x (commit here, see TRUNK-14).
@dkayiwa, looks like that a StandardRegimenTagHandlerSupport2_0 should be introduced that stops doing that?
@yadamz, the actual error was:
java.lang.NoClassDefFoundError: org/openmrs/order/RegimenSuggestion
at org.openmrs.module.htmlformentry.handler.StandardRegimenTagHandlerSupport1_10.createAttributeDescriptors(StandardRegimenTagHandlerSupport1_10.java:23)
As in: no class definition found for RegimenSuggestion, leading me to check that it indeed was nowhere to be found on Core master (or any 2.x branch). However I saw that it still existed on Core 1.12.x (see here).