Could not delete file text from the Apache Solr index. Check recent log messages.

Sounds like corrupt data? Can you make this reproducible?

I am seeing this issue when I save a taxonomy term with a new file attachment, but I don't see it when deleting attachments from taxonomy terms or updating a term with an attachment without changing anything. The file attachment does attach to the term in spite of the error.

EDIT: I am not seeing this error when submitting new file attachments on nodes - only on taxonomy terms (though I haven't done a lot of troubleshooting yet).

I added some logging around every call to entity_extract_ids(), and it appears the problem is occuring (in my case, at least) in the entity_extract_ids() call in _apachesolr_attachments_update_parent_entity() on line 556.

In entity_extract_ids() in common.inc, $info['entity keys']['bundle'] is either not set or is set to vocabulary_machine_name for nearly all of the entities I checked on term save, and where the bundle is set to vocabulary_machine_name, $entity->vocabulary_machine_name is set to a valid vocabulary machine name. In the final entity_extract_ids() call where the error occured, the bundle is set to 'type', which does not exist as a property of $entity (but $entity->vocabulary_machine_name is set, and the term id is the same as in previous calls).

At this point, I noticed that the $entity_type parameter for entity_extract_ids() is hardcoded to 'node' in _apachesolr_attachments_update_parent_entity(). I swapped it for $parent_entity_type, and I no longer get the error.

Removed line 556:
list($parent_entity_id, $parent_entity_vid, $parent_entity_bundle) = entity_extract_ids('node', $parent_entity);

Replaced with:
list($parent_entity_id, $parent_entity_vid, $parent_entity_bundle) = entity_extract_ids($parent_entity_type, $parent_entity);

At this point, I don't know what effect this change might have downstream, but I'm going to do a bit more testing and see if it might be safe to just swap all instances of hardcoded 'node' in calls to entity_extract_ids() with the appropriate variable. If it seems safe for my sites, I will submit a patch.

There were 6 calls to entity_extract_ids(), but only two of them appear to have had the parent entity type hardcoded to 'node'.

To fix the second hardcoded entity type, in function apachesolr-attachments_node_solr_document(), I replaced this line:
list($parent_entity_id, $parent_entity_vid, $parent_entity_bundle) = entity_extract_ids('node', $parent_entity);

With this line:
list($parent_entity_id, $parent_entity_vid, $parent_entity_bundle) = entity_extract_ids($env_id, $parent_entity);

NOTE: The comments for this function suggest that the third argument should be $entity_type, but it is named $env_id instead. I don't see the variable actually getting used anywhere in the function, and it appears to actually contain the entity type, so if this actually works, I will rename it when I submit a patch unless one of the module maintainers would prefer I do not rename it.

/**
* Builds the file-specific information for a Solr document.
*
* @param ApacheSolrDocument $document
* The Solr document we are building up.
* @param stdClass $entity
* The entity we are indexing.
* @param string $entity_type
* The type of entity we're dealing with.
*/
function apachesolr_attachments_node_solr_document(ApacheSolrDocument &$document, $parent_entity, $env_id) {

Search indexing appears to be running without generating errors, and I can save file attachments on taxonomy terms without generating errors, so here is a patch.

omaster - I suspect the hardcoded entity type in function apachesolr-attachments_node_solr_document() (comment #8) is more likely the cause of your problem rather than the one I posted in comment #5, so this patch may be worth trying if you have not already switched back to another module and the amount of content you have is relatively small (the site I need this for will take at least a full day to reindex when I move the fix to production). The patch addresses the hardcoded entity types in comments #5 and #8.

Seems like a possible duplicate of #2195095: Error when trying using a file field on entities on than nodes e.g. field collection item? (Well actually this issue is older, but that one is marked RTBC.)