On a OH 2.5.12, the Telegram binding worked fine after the initial install.
After changing the thing configuration (chat id), the binding produced a communication error and the following error in the log :
java.io.IOException: com.google.gson.JsonSyntaxException: java.lang.NumberFormatException: Expected an int but was <chat id> at line 2 column 396 path $.result[0].message.left_chat_member.id
Of course returning to the orignal chat id, the error stays the same.