I was experiencing this bug only when indexing. Which was making me think it's due to misconfiguration.
Drupal\search_api\SearchApiException while trying to
index items on index main:
Solr endpoint https://mysite:443/solr/main/ unreachable
It turns out, this error can happen when it actually has reached the server, but you try to do something that throws an error in SOLR.
[c:main s:shard2 r:core_main_node2 x:main]
multiple values encountered for non multiValued
field bs_mycustom_field: [true, true]
I was trying to add an array to a boolean in my custom processor!
It had worked previously but something changed where it was less forgiving.
I changed from this:
$mycustomfield->addValue(($data_item['status'] === "1"));
$mycustomfield->setValues([$data_item['status'] === "1"]);
in my custom SearchApiProcessor plugin (Drupal 8), then it indexed without issue.
The big takeaway here is, this error can be misleading! It did reach SOLR!