(WinRAR isn't used to read RAR archives so it doesn't matter if it's installed or not.)
Does the problem happen with all RAR archives or just some?
Does it matter where the archives are stored? (e.g. Do they work if you copy them to your desktop folder?)
Are they multi-part RARs or single?
Can you open the same archives in WinRAR or other tools without problems?
If you search your C:\ drive for copies of unrar*.dll, are there any in system locations like C:\Windows\System32?
Workaround: Type /home/VFSPlugins into Opus, then delete the unrar.dll or unrar64.dll file, and then fully exit Opus. Opus will fall back on using the old method for RAR extraction (via 7z.dll instead of unrar.dll), and you'll just lose RAR 5.0 support and possibly some performance when extracting RAR files.