I read this question that suggests that modals are disruptive, but I want to know if they are good to use with server errors, when the user has finished their task.
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Not just about modal errors, but errors in general:
If all of these questions are a 'no', then there is no point in explicitly showing the error to the user. A simple notification that something is going on and is not finished in a corner somewhere is sufficient.
The other answer you mention states that modal dialogs distract the user. Therefore it is inadvisable to use them during the normal user workflow. However, if there is an event that is both (a) uncommon, and (b) important, then modal dialogs are entirely appropriate.
Server errors may fall into this category depending on how infrequent and important they are. If it is something that most users will see during the regular coarse of using your interface (like the network connection going away), then it is probably best to handle the error inside of the page. If something seriously bad might happen (like data loss) as a result of not noticing the message, then a modal is appropriate.