**Steps to replicate the issue** (include links if applicable):
1. Start a translation for a section (probably lead section) that contains a block template for which cxserver returns 500 error. For example the infobox inside this lead section:
https://el.m.wikipedia.org/w/index.php?title=Special:ContentTranslation&page=Ronald%20Vargas&from=en&to=el&sx=true
2. Proceed to the "Pick a sentence" step to actually translate the content.
3. Click on that block template
Screencast of the issue in greek (el) wiki:
{F35818890}
**What happens?**:
The loading indicator inside the block template adaptation card at the bottom, shows up forever.
**What should have happened instead?**:
A message indicating that the adaptation of this template failed should be displayed to the user, as described in T295973.