404 means their server responded saying the requested path doesn’t exist. This is not a server problem and wouldn’t be fixed by the solution they posted. Ensure the URL you’re using is correct (if they recently changed it to fix the issue they mentioned), though I’m not seeing a 404 when trying to connect now.
16-9-11 23:19:44.3|Warn|HttpClient|HTTP Error - Res: [GET] https://tabula-rasa.pw/api?t=caps&apikey=(removed) 404.NotFound
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>404 Not Found</title>
</head><body>
<h1>Not Found</h1>
<p>The requested URL /api was not found on this server.</p>
<hr>
<address>Apache/2.4.18 (Ubuntu) Server at www.tabula-rasa.pw Port 80</address>
</body></html>
16-9-11 23:19:44.5|Warn|HttpClient|HTTP Error - Res: [GET] https://tabula-rasa.pw/api?t=tvsearch&cat=5030,5040&extended=1&apikey=(removed)&offset=0&limit=100: 404.NotFound
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>404 Not Found</title>
</head><body>
<h1>Not Found</h1>
<p>The requested URL /api was not found on this server.</p>
<hr>
<address>Apache/2.4.18 (Ubuntu) Server at www.tabula-rasa.pw Port 80</address>
</body></html>
16-9-11 23:19:44.5|Warn|Newznab|Unable to connect to indexer
[v2.0.0.4323] NzbDrone.Common.Http.HttpException: HTTP request failed: [404:NotFound] [GET] at [https://tabula-rasa.pw/api?t=tvsearch&cat=5030,5040&extended=1&apikey=(removed)&offset=0&limit=100]
at NzbDrone.Common.Http.HttpClient.Execute(HttpRequest request) in M:\BuildAgent\work\6c3239faf2b92630\src\NzbDrone.Common\Http\HttpClient.cs:line 101
at NzbDrone.Core.Indexers.HttpIndexerBase`1.FetchIndexerResponse(IndexerRequest request) in M:\BuildAgent\work\6c3239faf2b92630\src\NzbDrone.Core\Indexers\HttpIndexerBase.cs:line 298
at NzbDrone.Core.Indexers.HttpIndexerBase`1.FetchPage(IndexerRequest request, IParseIndexerResponse parser) in M:\BuildAgent\work\6c3239faf2b92630\src\NzbDrone.Core\Indexers\HttpIndexerBase.cs:line 284
at NzbDrone.Core.Indexers.HttpIndexerBase`1.TestConnection() in M:\BuildAgent\work\6c3239faf2b92630\src\NzbDrone.Core\Indexers\HttpIndexerBase.cs:line 311
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>404 Not Found</title>
</head><body>
<h1>Not Found</h1>
<p>The requested URL /api was not found on this server.</p>
<hr>
<address>Apache/2.4.18 (Ubuntu) Server at www.tabula-rasa.pw Port 80</address>
</body></html>
16-9-11 23:19:44.6|Warn|NzbDroneErrorPipeline|Invalid request Validation failed:
-- Unable to connect to indexer, check the log for more details
You need to use https://www.tabula-rasa.pw as the URL (add www.), it looks like there is a redirect to www. which your browser follows, but Sonarr doesn’t.