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Hello,(Adapted from a comment on https://forum.plantuml.net/11089)
As first workaround, you can use the unicode notation:
@startmindmap * Class Templates **:template <typename T> class cname{ void f1()<U+003B> ... };; @endmindmap
[http://www.plantuml.com/plantuml/umla/SoWkIImgoStCIybDBE3Yqb9mpaaiBbO8ISqjo4aiIItcqjAoAe5o56nAAWjIyn9pKnL2xBYImGgJGTnghhByp1I5D4CDJPjGRGC3OoSxBZqzFQvQQsikBYw7rBmKg0K0]
See also:Special characters on https://plantuml.com/en/creole