Thanks for your feedback above which shared more information and glad to know that your issue is resolved now! Since our forum has the policy that The question author cannot accept their own answer. They can only accept answers by others, and according to the scenario introduced here: Answering your own questions on Microsoft Q&A
I would make a brief summary of this post so that other forum members could easily find useful information here:
[Cannot Send approval "An unexpected server error occurred." - Summary]
Issue Symptom:
Cannot send an approval from the MS Teams app. Whenever i click on Send, I get the error in the subject. This only happens template's which fall under the same group
Solution:
One of the approvers has left the company and her sign-in was blocked. Removing her from the approver's list solved it.
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