diff --git a/src/servers/model/server/query.ts b/src/servers/model/server/query.ts
index bba3bf51e541fd67ca8bdbb020c13bf518d10444..a134c16826c518590f75535f5280cc365d49f29a 100644
--- a/src/servers/model/server/query.ts
+++ b/src/servers/model/server/query.ts
@@ -6,7 +6,6 @@
 
 import { Column } from 'mol-data/db';
 import { CifWriter } from 'mol-io/writer/cif';
-import Writer from 'mol-io/writer/writer';
 import { StructureQuery, StructureSelection } from 'mol-model/structure';
 import { encode_mmCIF_categories } from 'mol-model/structure/export/mmcif';
 import { now, Progress } from 'mol-task';
@@ -26,12 +25,11 @@ export interface Stats {
 
 const perf = new PerformanceMonitor();
 
-export async function resolveJob(job: Job, writer: Writer) {
+export async function resolveJob(job: Job): Promise<CifWriter.Encoder<any>> {
     ConsoleLogger.logId(job.id, 'Query', 'Starting.');
 
     const wrappedStructure = await getStructure(job);
 
-    let startedWriting = false;
     try {
         const encoder = CifWriter.createEncoder({ binary: job.responseFormat.isBinary, encoderName: `ModelServer ${Version}` });
         perf.start('query');
@@ -54,8 +52,6 @@ export async function resolveJob(job: Job, writer: Writer) {
         // encoder.setFilter();
         perf.end('encode');
 
-        ConsoleLogger.logId(job.id, 'Query', 'Encoded.');
-
         const stats: Stats = {
             structure: wrappedStructure,
             queryTimeMs: perf.time('query'),
@@ -64,26 +60,21 @@ export async function resolveJob(job: Job, writer: Writer) {
 
         encoder.writeCategory(_model_server_stats, [stats]);
         encoder.encode();
-        startedWriting = true;
-        encoder.writeTo(writer);
-        ConsoleLogger.logId(job.id, 'Query', 'Written.');
+        ConsoleLogger.logId(job.id, 'Query', 'Encoded.');
+        return encoder;
     } catch (e) {
         ConsoleLogger.errorId(job.id, e);
-        if (!startedWriting) {
-            doError(job, writer, e);
-        } else {
-            ConsoleLogger.errorId(job.id, 'Error was not relayed to the user because it happened during "write".');
-        }
+        return doError(job, e);
     }
 }
 
-function doError(job: Job, writer: Writer, e: any) {
+function doError(job: Job, e: any) {
     const encoder = CifWriter.createEncoder({ binary: job.responseFormat.isBinary, encoderName: `ModelServer ${Version}` });
     encoder.writeCategory(_model_server_result, [job]);
     encoder.writeCategory(_model_server_params, [job]);
     encoder.writeCategory(_model_server_error, ['' + e]);
     encoder.encode();
-    encoder.writeTo(writer);
+    return encoder;
 }
 
 const maxTime = Config.maxQueryTimeInMs;
diff --git a/src/servers/model/server/structure-wrapper.ts b/src/servers/model/server/structure-wrapper.ts
index 2190b602ab3d752a2d9427ce9a16e45c341d574f..72bc68992d12d99716441beb557ab8ad100d6c38 100644
--- a/src/servers/model/server/structure-wrapper.ts
+++ b/src/servers/model/server/structure-wrapper.ts
@@ -85,7 +85,7 @@ async function parseCif(data: string|Uint8Array) {
 async function readStructure(key: string, sourceId: string, entryId: string) {
     const filename = sourceId === '_local_' ? entryId : Config.mapFile(sourceId, entryId);
     if (!filename) throw new Error(`Cound not map '${key}' to a valid filename.`);
-    if (!fs.existsSync(filename)) throw new Error(`Could not map '${key}' to an existing file.`);
+    if (!fs.existsSync(filename)) throw new Error(`Could not find source file for '${key}'.`);
 
     perf.start('read');
     let data;
diff --git a/src/servers/model/server/web-api.ts b/src/servers/model/server/web-api.ts
index 61eee63eb68b502e932cd31ea5fb0a641fd442a6..ca96bbbe570eed8a552621917d6de3a93ee1791e 100644
--- a/src/servers/model/server/web-api.ts
+++ b/src/servers/model/server/web-api.ts
@@ -18,9 +18,9 @@ function makePath(p: string) {
 
 function wrapResponse(fn: string, res: express.Response) {
     const w = {
-        doError(this: any, code = 404) {
+        doError(this: any, code = 404, message = 'Not Found.') {
             if (!this.headerWritten) {
-                res.writeHead(code);
+                res.status(code).send(message);
                 this.headerWritten = true;
             }
             this.end();
@@ -66,33 +66,34 @@ async function processNextJob() {
 
     const filenameBase = `${job.entryId}_${job.queryDefinition.name.replace(/\s/g, '_')}`
     const writer = wrapResponse(job.responseFormat.isBinary ? `${filenameBase}.bcif` : `${filenameBase}.cif`, response);
+
     try {
+        const encoder = await resolveJob(job);
         writer.writeHeader(job.responseFormat.isBinary);
-        await resolveJob(job, writer);
+        encoder.writeTo(writer);
     } catch (e) {
         ConsoleLogger.errorId(job.id, '' + e);
-        // TODO: add some error?
-        writer.doError(404);
+        writer.doError(404, '' + e);
     } finally {
         writer.end();
+        ConsoleLogger.logId(job.id, 'Query', 'Finished.');
         setImmediate(processNextJob);
     }
 }
 
 function mapQuery(app: express.Express, queryName: string, queryDefinition: QueryDefinition) {
-    app.get(makePath(':entryId/' + queryName), async (req, res) => {
+    app.get(makePath(':entryId/' + queryName), (req, res) => {
         ConsoleLogger.log('Server', `Query '${req.params.entryId}/${queryName}'...`);
 
         if (JobManager.size >= Config.maxQueueLength) {
-            // TODO use proper code: server busy
-            res.writeHead(404);
+            res.status(503).send('Too many queries, please try again later.');
             res.end();
             return;
         }
 
         const jobId = JobManager.add('pdb', req.params.entryId, queryName, req.query);
         responseMap.set(jobId, res);
-        processNextJob();
+        if (JobManager.size === 1) processNextJob();
     });
 }
 
diff --git a/src/servers/model/test.ts b/src/servers/model/test.ts
index 330e31780b20c15c4bbf818fd8cb5b24638009c4..882c18b6afc6095f813f4f12c2e5e48bd07b0d3a 100644
--- a/src/servers/model/test.ts
+++ b/src/servers/model/test.ts
@@ -36,8 +36,9 @@ function wrapFile(fn: string) {
 async function run() {
     try {
         const request = createJob('_local_', 'e:/test/quick/1cbs_updated.cif', 'residueInteraction', { label_comp_id: 'REA' });
+        const encoder = await resolveJob(request);
         const writer = wrapFile('e:/test/mol-star/1cbs_full.cif');
-        await resolveJob(request, writer);
+        encoder.writeTo(writer);
         writer.end();
     } finally {
         StructureCache.expireAll();